1) General Questions of SQL SERVER
What is RDBMS?
Relational Data Base Management Systems (RDBMS) are
database management systems that maintain data records and indices in tables.
Relationships may be created and maintained across and among the data and
tables. In a relational database, relationships between data items are
expressed by means of tables. Interdependencies among these tables are
expressed by data values rather than by pointers. This allows a high degree of
data independence. An RDBMS has the capability to recombine the data items from
different files, providing powerful tools for data usage. ( Read More Here)
What are the properties of the Relational tables?
Relational
tables have six properties:
Values are atomic.
Column values are of the same kind.
Each row is unique.
The sequence of columns is insignificant.
The sequence of rows is insignificant.
Each column must have a unique name.
What is Normalization?
Database normalization is a data design and
organization process applied to data structures based on rules that help
building relational databases. In relational database design, the process of
organizing data to minimize redundancy is called normalization. Normalization
usually involves dividing a database into two or more tables and defining
relationships between the tables. The objective is to isolate data so that
additions, deletions, and modifications of a field can be made in just one
table and then propagated through the rest of the database via the defined
relationships.
What is De‐normalization?
De‐normalization is the
process of attempting to optimize the performance of a database by adding
redundant data. It is sometimes necessary because current DBMSs implement the
relational model poorly. A true relational DBMS would allow for a fully
normalized database at the logical level, while providing physical storage of
data that is tuned for high performance. De‐normalization
is a technique to move from higher to lower normal forms of database modeling
in order to speed up database access.
What are different normalization forms?
1NF: Eliminate Repeating Groups
Make a separate table for each set of related
attributes, and give each table a primary key. Each field contains at most one
value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi‐valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
If attributes do not contribute to a description of
the key, remove them to a separate table. All attributes must be directly
dependent on the primary key. ( Read More Here)
BCNF: Boyce‐Codd Normal Form
If
there are non‐trivial dependencies between candidate key attributes,
separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m
relationships that are not directly related.
5NF: Isolate Semantically Related Multiple
Relationships
There may be practical constrains on information that
justify separating logically related many‐to‐many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as
expressed in Object Role Model notation.
DKNF: Domain‐Key Normal Form
A model free from all modification anomalies is said
to be in DKNF.
Remember, these normalization guidelines are
cumulative. For a database to be in 3NF, it must first fulfill all the criteria
of a 2NF and 1NF database.
What is Stored Procedure?
A stored procedure is a named group of SQL statements
that have been previously created and stored in the server database. Stored procedures
accept input parameters so that a single procedure can be used over the network
by several clients using different input data. And when the procedure is
modified, all clients automatically get the new version. Stored procedures
reduce network traffic and improve performance. Stored procedures can be used
to help ensure the integrity of the database.
What is Trigger?
A trigger is a SQL procedure that initiates an action
when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and
managed by the DBMS. Triggers are used to maintain the referential integrity of
data by changing the data in a systematic fashion. A trigger cannot be called
or executed; DBMS automatically fires the trigger as a result of a data
modification to the associated table. Triggers can be viewed as similar to
stored procedures in that both consist of procedural logic that is stored at
the database level. Stored procedures, however, are not event‐drive and are not attached to a specific table as
triggers are. Stored procedures are explicitly executed by invoking a CALL to
the procedure while triggers are implicitly executed. In addition, triggers can
also execute stored procedures.
Nested Trigger: A trigger can also contain INSERT,
UPDATE and DELETE logic within itself, so when the trigger is fired because of
data modification it can also cause another data modification, thereby firing
another trigger. A trigger that contains data modification logic within itself
is called a nested trigger.
What
is View?
A simple view can be thought of as a subset of a
table. It can be used for retrieving data, as well as updating or deleting
rows. Rows updated or deleted in the view are updated or deleted in the table
the view was created with. It should also be noted that as data in the original
table changes, so does data in the view, as views are the way to look at part
of the original table. The results of using a view are not permanently stored
in the database. The data accessed through a view is actually constructed using
standard T‐SQL select command and can come from one to many
different base tables or even other views.
What is Index?
An index is a physical structure containing pointers
to the data. Indices are created in an existing table to locate rows more
quickly and efficiently. It is possible to create an index on one or more
columns of a table, and each index is given a name. The users cannot see the
indexes; they are just used to speed up queries. Effective indexes are one of
the best ways to improve performance in a database application. A table scan
happens when there is no index available to help a query. In a table scan SQL
Server examines every row in the table to satisfy the query results. Table
scans are sometimes unavoidable, but on large tables, scans have a terrific
impact on performance.
What is a Linked Server?
Linked Servers is a concept in SQL Server by which we
can add other SQL Server to a Group and query both the SQL Server dbs using T‐SQL Statements. With a linked server, you can create
very clean, easy to follow, SQL statements that allow remote data to be
retrieved, joined and combined with local data. Stored Procedure
sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server.
What is Cursor?
Cursor is a database object used by applications to
manipulate data in a set on a row‐by‐ row basis, instead of the typical SQL commands that
operate on all the rows in the set at one time.
In order to work with a cursor we need to perform some
steps in the following order:
Declare cursor
Open cursor
Fetch row from the cursor
Process fetched row
Close cursor
Deallocate cursor ( Read More Here)
What
is Collation?
Collation refers to a set of rules that determine how
data is sorted and compared. Character data is sorted using rules that define
the correct character sequence, with options for specifying case sensitivity,
accent marks, kana character types and character width. ( Read More Here)
What
is Difference between Function and Stored Procedure?
UDF can be used in the SQL statements anywhere in the
WHERE/HAVING/SELECT section where as Stored procedures cannot be. UDFs that
return tables can be treated as another rowset. This can be used in JOINs with
other tables. Inline UDF's can be thought of as views that take parameters and
can be used in JOINs and other Rowset operations.
What is sub‐query? Explain properties of sub‐query?
Sub‐queries are often
referred to as sub‐selects, as they allow a SELECT statement to be
executed arbitrarily within the body of another SQL statement. A sub‐query is executed by enclosing it in a set of
parentheses. Sub‐queries are generally used to return a single row as
an atomic value, though they may be used to compare values against multiple
rows with the IN keyword.
A subquery is a SELECT statement that is nested within
another T‐SQL statement. A subquery SELECT statement if executed
independently of the T‐SQL statement, in which it is nested, will return a
resultset. Meaning a subquery SELECT statement can standalone and is not
depended on the statement in which it is nested. A subquery SELECT statement
can return any number of values, and can be found in, the column list of a
SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a T‐SQL statement. A Subquery can also be used as a
parameter to a function call. Basically a subquery can be used anywhere an
expression can be used.
What are different Types of Join?
Cross Join
A cross join that does not have a WHERE clause
produces the Cartesian product of the tables involved in the join. The size of
a Cartesian product result set is the number of rows in the first table
multiplied by the number of rows in the second table. The common example is
when company wants to combine each product with a pricing table to analyze each
product at each price.
Inner Join
A join that displays only the rows that have a match
in both joined tables is known as inner Join. This is the default type of join
in the Query and View Designer.
Outer Join
A
join that includes rows even if they do not have related rows in the joined
table is an Outer Join. You can create three different outer join to specify
the unmatched rows to be included:
Left Outer Join: In Left Outer Join all rows in the first‐named
table i.e. "left" table, which appears leftmost in the JOIN
clause are included. Unmatched rows in the right table do not appear.
Right Outer Join: In Right Outer Join all rows in the second‐named table i.e. "right" table, which
appears rightmost in the JOIN clause are included. Unmatched rows in the left
table are not included.
Full Outer Join: In Full Outer Join all rows in all joined tables are included, whether
they are matched or not.
Self Join
This is a particular case when one table joins to
itself, with one or two aliases to avoid confusion. A self join can be of any
type, as long as the joined tables are the same. A self join is rather unique
in that it involves a relationship with only one table. The common example is
when company has a hierarchal reporting structure whereby one member of staff
reports to another. Self Join can be Outer Join or Inner Join.
What are primary keys and foreign keys?
Primary keys are the unique identifiers for each row.
They must contain unique values and cannot be null. Due to their importance in
relational databases, Primary keys are the most fundamental of all keys and
constraints. A table can have only one Primary key.
Foreign keys are both a method of ensuring data
integrity and a manifestation of the relationship between tables.
What is User Defined Functions? What kind of User‐Defined Functions can be created?
User‐Defined Functions allow
defining its own T‐SQL functions that can accept 0 or more parameters and
return a single scalar data value or a table data type.
Different Kinds of User‐Defined
Functions created are:
Scalar User‐Defined Function
A Scalar user‐defined
function returns one of the scalar data types. Text, ntext, image and timestamp
data types are not supported. These are the type of user‐defined functions that most developers are used to in
other programming languages. You pass in 0 to many parameters and you get a
return value.
Inline Table‐Value User‐Defined Function
An Inline Table‐Value
user‐defined function returns a table data type and is an
exceptional alternative to a view as the user‐defined
function can pass parameters into a T‐SQL
select command and in essence provide us with a parameterized, non‐updateable view of the underlying tables.
Multi‐statement Table‐Value User‐Defined Function
A Multi‐Statement
Table‐Value user‐defined
function returns a table and is also an exceptional alternative to a view as
the function can support multiple T‐SQL
statements to build the final result where the view is limited to a single
SELECT statement. Also, the ability to pass parameters into a TSQL select
command or a group of them gives us the capability to in essence create a
parameterized, non‐updateable view of the data in the underlying tables.
Within the create function command you must define the table structure that is
being returned. After creating this type of user‐defined
function, It can be used in the FROM clause of a T‐SQL command unlike the behavior found when using a
stored procedure which can also return record sets
What is Identity?
Identity (or AutoNumber) is a column that
automatically generates numeric values. A start and increment value can be set,
but most DBA leave these at 1. A GUID column also generates numbers; the value
of this cannot be controlled. Identity/GUID columns do not need to be indexed.
What is DataWarehousing?
Subject‐oriented,
meaning that the data in the database is organized so that all the data
elements relating to the same real‐world
event or object are linked together;
Time‐variant, meaning that
the changes to the data in the database are tracked and recorded so that
reports can be produced showing changes over time;
Non‐volatile, meaning that
data in the database is never over‐written
or deleted, once committed, the data is static, read‐only, but retained for future reporting.
Integrated, meaning that the database contains data
from most or all of an organization's operational applications, and that this
data is made consistent.
2) Common Questions Asked
Which TCP/IP port does SQL Server run on? How can it
be changed?
SQL Server runs on port 1433. It can be changed from
the Network Utility TCP/IP properties –> Port number, both on client and the
server.
A clustered index is a special type of index that reorders the way records in the table
are physically stored. Therefore table can have only one clustered
index. The leaf nodes of a clustered index contain the data pages.
A non clustered index is a special type of index in which the logical order
of the index does not match the physical stored order of the rows on
disk. The leaf node of a non clustered index does not consist of the data
pages. Instead, the leaf nodes contain index rows.
What
are the different index configurations a table can have?
A table can have one of the following index
configurations:
No indexes
A clustered index
A clustered index and many nonclustered indexes
A nonclustered index
Many nonclustered indexes
What are different types of Collation Sensitivity?
Case sensitivity ‐ A
and a, B and b, etc.
Accent sensitivity ‐ a
and á, o and ó, etc.
Kana Sensitivity ‐
When Japanese kana characters Hiragana and Katakana are treated differently, it
is called Kana sensitive.
Width sensitivity ‐ A
single‐byte character (half‐width)
and the same character represented as a double‐byte
character (full‐width) are treated differently than it is width
sensitive. ( Read More Here)
What is OLTP (Online Transaction Processing)?
In OLTP ‐
online transaction processing systems relational database design use the
discipline of data modeling and generally follow the Codd rules of data
normalization in order to ensure absolute data integrity. Using these rules
complex information is broken down into its most simple structures (a table)
where all of the individual atomic level elements relate to each other and
satisfy the normalization rules.
What's the difference between a primary key and a
unique key?
Both primary key and unique key enforces uniqueness of
the column on which they are defined. But by default primary key creates a
clustered index on the column, where are unique creates a nonclustered index by
default. Another major difference is that, primary key doesn't allow NULLs, but
unique key allows one NULL only.
What is difference between DELETE & TRUNCATE
commands?
Delete command removes the rows from a table based on
the condition that we provide with a WHERE clause. Truncate will actually
remove all the rows from a table and there will be no data in the table after
we run the truncate command.
TRUNCATE
TRUNCATE is faster and uses fewer system and
transacton log resources than DELETE.
TRUNCATE removes the data by deallocating the data
pages used to store the table’s data, and only the page deallocations are
recorded in the transaction log.
TRUNCATE removes all rows from a table, but the table
structure, its columns, constraints, indexes and so on, remains. The counter
used by an identity for new rows is reset to the seed for the column.
You cannot use TRUNCATE TABLE on a table referenced by
a FOREIGN KEY constraint. Because TRUNCATE TABLE is not logged, it cannot
activate a trigger.
TRUNCATE cannot be rolled back.
TRUNCATE is DDL Command.
TRUNCATE Resets identity of the table
DELETE
DELETE removes rows one at a time and records an entry
in the transaction log for each deleted row.
If you want to retain the identity counter, use DELETE
instead. If you want to remove table definition and its data, use the DROP
TABLE statement.
DELETE Can be used with or without a WHERE clause
DELETE Activates Triggers.
DELETE can be rolled back.
DELETE is DML Command.
DELETE does not reset identity of the table. ( Read More Here)
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing
of data has occurred. If a large amount of deletions any modification or Bulk
Copy into the tables has occurred, it has to update the indexes to take these
changes into account. UPDATE_STATISTICS updates the indexes on these tables
accordingly.
What is the difference between a HAVING CLAUSE and a
WHERE CLAUSE?
They specify a search condition for a group or an
aggregate. But the difference is that HAVING can be used only with the SELECT
statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not
used, HAVING behaves like a WHERE clause. Having Clause is basically used only
with the GROUP BY function in a query whereas WHERE Clause is applied to each
row before they are part of the GROUP BY function in a query.
What are the properties and different Types of Sub‐Queries?
Properties of Sub‐Query
A sub‐query must be enclosed
in the parenthesis.
A sub‐query must be put in the
right hand of the comparison operator, and
A sub‐query cannot contain an
ORDER‐BY clause.
A query can contain more than one sub‐query.
Types
of Sub‐query
Single‐row sub‐query, where the sub‐query
returns only one row.
Multiple‐row
sub‐query, where the sub‐query
returns multiple rows,. and
Multiple column sub‐query,
where the sub‐query returns multiple columns
What is SQL Profiler?
SQL Profiler is a graphical tool that allows system
administrators to monitor events in an instance of Microsoft SQL Server. You
can capture and save data about each event to a file or SQL Server table to
analyze later. For example, you can monitor a production environment to see
which stored procedures are hampering performances by executing too slowly.
Use SQL Profiler to monitor only the events in which
you are interested. If traces are becoming too large, you can filter them based
on the information you want, so that only a subset of the event data is
collected. Monitoring too many events adds overhead to the server and the
monitoring process and can cause the trace file or trace table to grow very
large, especially when the monitoring process takes place over a long period of
time.
What are the authentication modes in SQL Server? How
can it be changed?
Windows mode and Mixed Mode ‐ SQL & Windows.
To change authentication mode in SQL Server click
Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run
SQL Enterprise Manager from the Microsoft SQL Server program group. Select the
server then from the Tools menu select SQL Server Configuration Properties, and
choose the Security page.
Which command using Query Analyzer will give you the
version of SQL server and operating system?
SELECT SERVERPROPERTY ('productversion'),
SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition').
What is SQL Server Agent?
SQL Server agent plays an important role in the day‐to‐day tasks of a database
administrator (DBA). It is often overlooked as one of the main tools for SQL
Server management. Its purpose is to ease the implementation of tasks for the
DBA, with its full‐ function scheduling engine, which allows you to
schedule your own jobs and scripts
Can a stored procedure call itself or recursive stored
procedure? How much level SP nesting is possible?
Yes.
Because Transact‐SQL supports recursion, you can write stored
procedures that call themselves. Recursion can be defined as a method of
problem solving wherein the
solution is arrived at by repetitively applying it to
subsets of the problem. A common application of recursive logic is to perform
numeric computations that lend themselves to repetitive evaluation by the same
processing steps. Stored procedures are nested when one stored procedure calls
another or executes managed code by referencing a CLR routine, type, or
aggregate. You can nest stored procedures and managed code references up to 32
levels.
What is Log Shipping?
Log shipping is the process of automating the backup
of database and transaction log files on a production SQL server, and then
restoring them onto a standby server. Enterprise Editions only supports log
shipping. In log shipping the transactional log file from one server is
automatically updated into the backup database on the other server. If one
server fails, the other server will have the same db and can be used this as
the Disaster Recovery plan. The key feature of log shipping is that it will
automatically backup transaction logs throughout the day and automatically
restore them on the standby server at defined interval.
Name 3 ways to get an accurate count of the number of
records in a table?
SELECT * FROM table1
SELECT COUNT(*) FROM table1
SELECT rows FROM sysindexes WHERE id =
OBJECT_ID(table1) AND indid < 2
What does it mean to have QUOTED_IDENTIFIER ON? What
are the implications of having it OFF?
When SET QUOTED_IDENTIFIER is ON, identifiers can be
delimited by double quotation marks, and literals must be delimited by single
quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be
quoted and must follow all Transact‐SQL
rules for identifiers. ( Read More Here)
What is the difference between a Local and a Global
temporary table?
A local temporary table exists only for the duration
of a connection or, if defined inside a compound statement, for the duration of
the compound statement.
A global temporary table remains in the database
permanently, but the rows exist only within a given connection. When connection
is closed, the data in the global temporary table disappears. However, the
table definition remains with the database for access when database is opened
next time.
What is the STUFF function and how does it differ from
the REPLACE function?
STUFF function is used to overwrite existing
characters. Using this syntax, STUFF (string_expression, start, length,
replacement_characters), string_expression is the string that will have
characters substituted, start is the starting position, length is the number of
characters in the string that are substituted, and replacement_characters are
the new characters interjected into the string. REPLACE function to replace
existing characters of all occurrences. Using the syntax REPLACE
(string_expression, search_string, replacement_string), where every incidence
of search_string found in the string_expression will be replaced with
replacement_string.
What
is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a
row within a database table. Every table should have a primary key constraint
to uniquely identify each row and only one primary key constraint can be
created for each table. The primary key constraints are used to enforce entity
integrity.
What is UNIQUE KEY constraint?
A UNIQUE constraint enforces the uniqueness of the
values in a set of columns, so no duplicate values are entered. The unique key
constraints are used to enforce entity integrity as the primary key
constraints.
What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that
would destroy links between tables with the corresponding data values. A
foreign key in one table points to a primary key in another table. Foreign keys
prevent actions that would leave rows with foreign key values when there are no
primary keys with that value. The foreign key constraints are used to enforce
referential integrity.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that
can be placed in a column. The check constraints are used to enforce domain
integrity. ( Read More Here)
What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will
not accept null values. The not null constraints are used to enforce domain
integrity, as the check constraints.
How to get @@ERROR and @@ROWCOUNT at the same time?
If @@Rowcount is checked after Error checking
statement then it will have 0 as the value of @@Recordcount as it would have
been reset. And if @@Recordcount is checked before the error‐checking statement then @@Error would get reset. To
get @@error and @@rowcount at the same time do both in same statement and store
them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run
on regular or predictable cycles. User can schedule administrative tasks, such
as cube processing, to run during times of slow business activity. User can
also determine the order in which tasks run by creating job steps within a SQL
Server Agent job. E.g. back up database, Update Stats of Tables. Job steps give
user control over flow of execution. If one job fails, user can configure SQL
Server Agent to continue to run the remaining tasks or to stop execution.
What
are the advantages of using Stored Procedures?
Stored procedure can reduced network traffic and
latency, boosting application performance.
Stored procedure execution plans can be reused,
staying cached in SQL Server's memory, reducing server overhead.
Stored procedures help promote code reuse.
Stored procedures can encapsulate logic. You can
change stored procedure code without affecting clients.
Stored procedures provide better security to your
data.
What is a table called, if it has neither Cluster nor
Non‐cluster Index? What is it used for?
Unindexed table or Heap. Microsoft Press Books and
Book on Line (BOL) refers it as Heap. A heap is a table that does not have a
clustered index and, therefore, the pages are not linked by pointers. The IAM
pages are the only structures that link the pages in a table together.
Unindexed tables are good for fast storing of data. Many times it is better to
drop all indexes from table and then do bulk of inserts and to restore those
indexes after that.
Can SQL Servers linked to other servers like Oracle?
SQL Server can be linked to any server provided it has
OLE‐DB provider from Microsoft to allow a link. E.g.
Oracle has an OLE‐DB provider for oracle that Microsoft provides to add
it as linked server to SQL Server group
What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data
from tables and views. BCP does not copy the structures same as source to destination.
BULK INSERT command helps to import a data file into a database table or view
in a user‐specified format.
What command do we use to rename a db, a table and a
column?
To rename db
sp_renamedb ‘oldname’ , ‘newname’
If someone is using db it will not accept sp_renmaedb.
In that case first bring db to single user using sp_dboptions. Use sp_renamedb
to rename database. Use sp_dboptions to bring database to multi user mode.
E.g.
USE master; GO
EXEC sp_dboption AdventureWorks, 'Single User', True GO
EXEC sp_renamedb 'AdventureWorks', 'AdventureWorks_New'
GO
EXEC sp_dboption AdventureWorks, 'Single User', False GO
To rename Table
We can change the table name using sp_rename as
follows, sp_rename ‘oldTableName’ ‘newTableName’
E.g.
SP_RENAME ‘Table_First’, ‘Table_Last’ GO
To rename Column
The script for renaming any column :
sp_rename ‘TableName.[OldcolumnName]’,
‘NewColumnName’, ‘Column’ E.g.
sp_RENAME ‘Table_First.Name’, ‘NameChange’ , ‘COLUMN’ GO
What are sp_configure commands and set commands?
Use sp_configure to display or change server‐level settings. To change database‐level settings, use ALTER DATABASE. To change settings
that affect only the current user session, use the SET statement.
E.g.
sp_CONFIGURE ’show
advanced’, 0 GO
RECONFIGURE
GO sp_CONFIGURE GO
You
can run following command and check advance global configuration settings.
sp_CONFIGURE ’show
advanced’, 1 GO
RECONFIGURE
GO sp_CONFIGURE GO
How to implement one‐to‐one, one‐to‐many and many‐to‐many relationships while designing tables?
One‐to‐One relationship can be implemented as a single table
and rarely as two tables with primary and foreign key relationships. One‐to‐Many relationships are implemented
by splitting the data into two tables with primary key and foreign key
relationships. Many‐to‐Many relationships are
implemented using a junction table with the keys from both the tables forming
the composite primary key of the junction table.
What is an execution plan? When would you use it? How
would you view the execution plan?
An execution plan is basically a road map that
graphically or textually shows the data retrieval methods chosen by the SQL
Server query optimizer for a stored procedure or ad‐ hoc query and is a very useful tool for a developer
to understand the performance characteristics of a query or stored procedure
since the plan is the one that SQL Server will place in its cache and use to
execute the stored procedure or query. From within Query Analyzer is an option
called "Show Execution Plan" (located on the Query drop‐down menu). If this option is turned on it will
display query execution plan in separate window when query is ran again.
3) Questions of SQL SERVER 2008
What are the basic functions for master, msdb, model,
tempdb and resource databases?
The master database holds information for all
databases located on the SQL Server instance and is theglue that holds the
engine together. Because SQL Server cannot start without a functioning masterdatabase,
you must administer this database with care.
The msdb database stores information regarding
database backups, SQL Agent information, DTS packages, SQL Server jobs, and
some replication information such as for log shipping.
The tempdb holds temporary objects such as
global and local temporary tables and stored procedures.
The model
is essentially a template database used in the creation of any new user
database created in the instance.
The resoure Database is a read‐only database that contains all the system objects
that are included with SQL Server. SQL Server system objects, such as
sys.objects, are physically persisted in the Resource database, but they
logically appear in the sys schema of every database. The Resource database
does not contain user data or user metadata.
What is Service Broker?
Service Broker is a message‐queuing technology in SQL Server that allows
developers to integrate SQL Server fully into distributed applications. Service
Broker is feature which provides facility to SQL Server to send an
asynchronous, transactional message. it allows a database to send a message to
another database without waiting for the response, so the application will
continue to function if the remote database is temporarily unavailable. ( Read More Here)
Where SQL server user names and passwords are stored
in SQL server?
They get stored in System Catalog Views
sys.server_principals and sys.sql_logins.
What is Policy Management?
Policy Management in SQL SERVER 2008 allows you to define
and enforce policies for configuring and managing SQL Server across the
enterprise. Policy‐Based Management is configured in SQL Server
Management Studio (SSMS). Navigate to the Object Explorer and expand the
Management node and the Policy Management node; you will see the Policies,
Conditions, and Facets nodes.
What is Replication and Database Mirroring?
Database mirroring can be used with replication to
provide availability for the publication database. Database mirroring involves
two copies of a single database that typically reside on different computers.
At any given time, only one copy of the database is currently available to
clients which are known as the principal database. Updates made by clients to
the principal database are applied on the other copy of the database, known as
the mirror database. Mirroring involves applying the transaction log from every
insertion, update, or deletion made on the principal database onto the mirror
database.
What are Sparse Columns?
A sparse column is another tool used to reduce the
amount of physical storage used in a database. They are the ordinary columns
that have an optimized storage for null values. Sparse columns reduce the space
requirements for null values at the cost of more overhead to retrieve nonnull
values.
What
does TOP Operator Do?
The TOP operator is used to specify the number of rows
to be returned by a query. The TOP operator has new addition in SQL SERVER 2008
that it accepts variables as well as literal values and can be used with
INSERT, UPDATE, and DELETES statements.
What is CTE?
CTE is an abbreviation Common Table Expression. A
Common Table Expression (CTE) is an expression that can be thought of as a
temporary result set which is defined within the execution of a single SQL
statement. A CTE is similar to a derived table in that it is not stored
as an object and lasts only for the duration of the query.
What is MERGE Statement?
MERGE is a new feature that provides an efficient way
to perform multiple DML operations. In previous versions of SQL Server, we had
to write separate statements to INSERT, UPDATE, or DELETE data based on certain
conditions, but now, using MERGE statement we can include the logic of such
data modifications in one statement that even checks when the data is matched
then just update it and when unmatched then insert it. One of the most
important advantages of MERGE statement is all the data is read and processed
only once.
What is Filtered Index?
Filtered Index is used to index a portion of rows in a
table that means it applies filter on INDEX which improves query performance,
reduce index maintenance costs, and reduce index storage costs compared with
full‐table indexes. When we see an Index created with some
where clause then that is actually a FILTERED INDEX.
Which are new data types introduced in SQL SERVER
2008?
The GEOMETRY Type: The GEOMETRY data type is a system .NET common language runtime
(CLR) data type in SQL Server. This type represents data in a two‐dimensional Euclidean coordinate system.
The GEOGRAPHY Type: The GEOGRAPHY datatype’s functions are the same as with GEOMETRY.
The difference between the two is that when you specify GEOGRAPHY, you are
usually specifying points in terms of latitude and longitude.
New Date and Time Datatypes: SQL Server 2008 introduces four new datatypes related
to date and time: DATE, TIME, DATETIMEOFFSET, and DATETIME2.
DATE: The
new DATE type just stores the date itself. It is based on the Gregorian calendar
and handles years from 1 to 9999.
TIME: The
new TIME (n) type stores time with a range of 00:00:00.0000000 through 23:59:59.9999999.
The precision is allowed with this type. TIME supports seconds down to 100
nanoseconds. The n in TIME (n) defines this level of fractional second
precision, from 0 to 7 digits of precision.
The DATETIMEOFFSET Type: DATETIMEOFFSET (n) is the time‐zone‐aware version of a
datetime datatype. The name will appear less odd when you consider what it
really is: a date + a time + a time‐zone
offset. The offset is based on how far behind or ahead you are from Coordinated
Universal Time (UTC) time.
The DATETIME2 Type: It is an extension of the datetime type in earlier versions of SQL
Server. This new datatype has a date range covering dates from January 1 of
year 1 through December 31 of year 9999. This is a definite improvement over
the 1753 lower boundary of the datetime datatype. DATETIME2 not only includes
the larger date range, but also has a timestamp and the same fractional
precision that TIME type provides
What
are the Advantages of using CTE?
Using CTE improves the readability and makes
maintenance of complex queries easy.
The query can be divided into separate, simple,
logical building blocks which can be then used to build more complex CTEs until
final result set is generated.
CTE can be defined in functions, stored procedures,
triggers or even views.
After a CTE is defined, it can be used as a Table or a
View and can SELECT, INSERT, UPDATE or DELETE Data.
How can we rewrite sub‐queries into simple select statements or with joins?
Yes we can write using Common Table Expression (CTE).
A Common Table Expression (CTE) is an expression that can be thought of as a
temporary result set which is defined within the execution of a single SQL
statement. A CTE is similar to a derived table in that it is not stored as an
object and lasts only for the duration of the query.
E.g.
USE AdventureWorks GO
WITH EmployeeDepartment_CTE AS ( SELECT EmployeeID,DepartmentID,ShiftID
FROM HumanResources.EmployeeDepartmentHistory
)
SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID FROM HumanResources.Department ed
INNER JOIN EmployeeDepartment_CTE
ecte ON ecte.DepartmentID = ed.Department ID
GO
What is CLR?
In SQL Server 2008, SQL Server objects such as user‐defined functions can be created using such CLR
languages. This CLR language support extends not only to user‐defined functions, but also to stored procedures and
triggers. You can develop such CLR add‐ons
to SQL Server using Visual Studio 2008. ( Read More Here)
What are synonyms?
Synonyms give you the ability to provide alternate
names for database objects. You can alias object names; for example, using the
Employee table as Emp. You can also shorten names. This is especially useful
when dealing with three and four part names; for example, shortening
server.database.owner.object to object. ( Read More Here)
What is LINQ?
Language Integrated Query (LINQ) adds the ability to
query objects using .NET languages. The LINQ to SQL object/relational mapping
(O/RM) framework provides the following basic features:
Tools to create classes (usually called entities)
mapped to database tables
Compatibility with LINQ’s standard query operations
The DataContext class, with features such as entity
record monitoring, automatic SQL statement generation, record concurrency
detection, and much more
What is Isolation Levels?
Transactions specify an isolation level that defines
the degree to which one transaction must be isolated from resource or data
modifications made by other transactions. Isolation levels are described in
terms of which concurrency side‐effects, such as dirty
reads or phantom reads, are allowed.
Transaction isolation levels control:
Whether locks are taken when data is read, and what
type of locks are requested.
How long the read locks are held.
Whether a read operation referencing rows modified by
another transaction:
Blocks until the exclusive lock on the row is freed.
Retrieves the committed version of the row that
existed at the time the statement or transaction started.
Reads the uncommitted data modification. (Read More Here)
What
is use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle.
The EXCEPT query and MINUS query returns all rows in the first query that are
not returned in the second query. Each SQL statement within the EXCEPT query
and MINUS query must have the same number of fields in the result sets with
similar data types. ( Read More Here)
What is XPath?
XPath uses a set of expressions to select nodes to be
processed. The most common expression that you’ll use is the location path
expression, which returns back a set of nodes called a node set. XPath can use
both an unabbreviated and an abbreviated syntax. The following is the
unabbreviated syntax for a location path:
/axisName::nodeTest[predicate]/axisName::nodeTest[predicate]
What is NOLOCK?
Using the NOLOCK query optimizer hint is generally
considered good practice in order to improve concurrency on a busy system. When
the NOLOCK hint is included in a SELECT statement, no locks are taken when data
is read. The result is a Dirty Read, which means that another process could be
updating the data at the exact time you are reading it. There are no guarantees
that your query will retrieve the most recent data. The advantage to
performance is that your reading of data will not block updates from taking place,
and updates will not block your reading of data. SELECT statements take Shared
(Read) locks. This means that multiple SELECT statements are allowed
simultaneous access, but other processes are blocked from modifying the data.
The updates will queue until all the reads have completed, and reads requested
after the update will wait for the updates to complete. The result to your
system is delay (blocking). ( Read More Here)
How would you handle error in SQL SERVER 2008?
SQL Server now supports the use of TRY...CATCH
constructs for providing rich error handling. TRY...CATCH lets us build error
handling at the level we need, in the way we need to, by setting a region where
if any error occurs, it will break out of the region and head to an error
handler. The basic structure is as follows:
BEGIN TRY <code> END TRY
BEGIN CATCH <code>
END CATCH
So if any error occurs in the TRY block, execution is
diverted to the CATCH block, and the error can be dealt.
What
is RAISEERROR?
RaiseError generates an error message and initiates
error processing for the session. RAISERROR can either reference a user‐defined message stored in the sys.messages
catalog view or build a message dynamically. The message is returned as a
server error message to the calling application or to an associated CATCH block
of a TRY…CATCH construct. ( Read More Here)
How to rebuild Master Databse?
Master database is system database and it contains
information about running server’s configuration. When SQL Server 2005 is
installed it usually creates master, model, msdb, tempdb resource and
distribution system database by default. Only Master database is the one which
is absolutely must have database. Without Master database SQL Server cannot be
started. This is the reason it is extremely important to backup Master database.
To rebuild the Master database, Run Setup.exe, verify,
and repair a SQL Server instance, and rebuild the system databases. This
procedure is most often used to rebuild the master database for a
corrupted installation of SQL Server.
What is XML Datatype?
The xml data type lets you store XML documents
and fragments in a SQL Server database. An XML fragment is an XML instance that
is missing a single top‐level element. You can create columns and variables of
the xml type and store XML instances in them. The xml data type
and associated methods help integrate XML into the relational framework of SQL
Server.
What is Data Compression?
In SQL SERVE 2008 Data Compression comes in two
flavors:
Row Compression
Page Compression
Row Compression
Row
compression changes the format of physical storage of data. It minimize the
metadata (column information, length, offsets etc) associated with each record.
Numeric data types and fixed length strings are stored in variable‐length storage format, just like Varchar. ( Read More Here)
Page Compression
Page
compression allows common data to be shared between rows for a given page. Its
uses the following techniques to compress data:
Row compression.
Prefix Compression. For every column in a page
duplicate prefixes are identified. These prefixes are saved in compression
information headers (CI) which resides after page header. A reference number is
assigned to these prefixes and that reference number is replaced where ever
those prefixes are being used.
Dictionary Compression.
Dictionary compression searches for duplicate values
throughout the page and stores them in CI. The main difference between prefix
and dictionary compression is that prefix is only restricted to one column
while dictionary is applicable to the complete page.
What is use of DBCC Commands?
The Transact‐SQL
programming language provides DBCC statements that act as Database Console
Commands for SQL Server. DBCC commands are used to perform following tasks.
Maintenance tasks on database, index, or filegroup.
Tasks that gather and display various types of
information.
Validation operations on a database, table, index,
catalog, filegroup, or allocation of database pages.
Miscellaneous tasks such as enabling trace flags or
removing a DLL from memory.
How to find tables without Indexes?
Run
following query in Query Editor. USE <database_name>;
GO
SELECT SCHEMA_NAME(schema_id) AS schema_name ,name AS table_name
FROM sys.tables
WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0 ORDER BY schema_name, table_name;
GO
How to copy the tables, schema and views from one SQL
Server to another?
There are multiple ways to do this.
“Detach Database” from one server and “Attach
Database” to another server.
Manually script all the objects using SSMS and run the
script on new server.
Use Wizard of SSMS. (Read
More Here)
How
to copy data from one table to another table?
There are multiple ways to do this.
INSERT INTO SELECT
This method is used when table is already created in
the database earlier and data is to be inserted into this table from another
table. If columns listed in insert clause and select clause are same, they are
not required to list them.
SELECT INTO
This method is used when table is not created earlier
and needs to be created when data from one table is to be inserted into newly
created table from another table. New table is created with same data types as
selected columns.
What is Catalog Views?
Catalog views return information that is used by the
SQL Server Database Engine. Catalog Views are the most general interface to the
catalog metadata and provide the most efficient way to obtain, transform, and
present customized forms of this information. All user‐ available catalog metadata is exposed through catalog
views.
What is PIVOT and UNPIVOT?
A Pivot Table can automatically sort, count, and total
the data stored in one table or spreadsheet and create a second table
displaying the summarized data. The PIVOT operator turns the values of a
specified column into column names, effectively rotating a table.
UNPIVOT table is reverse of PIVOT Table.
What is Filestream?
Filestream allows you to store large objects in the
file system and have these files integrated within the database. It enables SQL
Server based applications to store unstructured data such as documents, images,
audios, videos etc. in the file system. FILESTREAM basically integrates the SQL
Server Database Engine with New Technology File System (NTFS); it basically
stores the data in varbinary (max) data type. Using this data type, the
unstructured data is stored in the NTFS file system and the SQL Server Database
Engine manages the link between the Filestream column and the actual file
located in the NTFS. Using Transact SQL statements users can insert, update,
delete and select the data stored in FILESTREAM enabled tables.
What is Dirty Read ?
A dirty read occurs when two operations say, read and
write occurs together giving the incorrect or unedited data. Suppose, A has
changed a row, but has not committed the changes. B reads the uncommitted data
but his view of the data may be wrong so that is Dirty Read.
What
is SQLCMD?
sqlcmd is enhanced version of the isql and osql and it
provides way more functionality than other two options. In other words sqlcmd
is better replacement of isql (which will be deprecated eventually) and osql
(not included in SQL Server 2005 RTM). sqlcmd can work two modes ‐ i) BATCH and ii) interactive modes. ( Read More)
What is Aggregate Functions?
Aggregate functions perform a calculation on a set of
values and return a single value. Aggregate functions ignore NULL values except
COUNT function. HAVING clause is used, along with GROUP BY, for filtering query
using aggregate values.
Following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING,
VAR, MAX, VARP
What do you mean by Table Sample?
TABLESAMPLE allows you to extract a sampling of rows
from a table in the FROM clause. The rows retrieved are random and they are not
in any order. This sampling can be based on a percentage of number of rows. You
can use TABLESAMPLE when only a sampling of rows is necessary for the
application instead of a full result set. ( Read More Here)
What is Row_Number()?
ROW_NUMBER() returns a column as an expression that
contains the row’s number within the result set. This is only a number used in
the context of the result set, if the result changes, the ROW_NUMBER() will
change.
What are Ranking Functions?
Ranking functions return a ranking value for each row
in a partition. All the ranking functions are non‐deterministic.
Different Ranking functions are:
ROW_NUMBER () OVER ([<partition_by_clause>]
<order_by_clause>)
Returns the sequential number of a row within a
partition of a result set, starting at 1 for the first row in each partition.
RANK () OVER ([<partition_by_clause>]
<order_by_clause>)
Returns the rank of each row within the partition of a
result set.
DENSE_RANK () OVER ([<partition_by_clause>]
<order_by_clause>)
Returns the rank of rows within the partition of a
result set, without any gaps in the ranking. ( Read More Here )
What
is the difference between UNION and UNION ALL?
UNION
The UNION command is used to select related information
from two tables, much like the JOIN command. However, when using the UNION
command all selected columns need to be of the same data type. With UNION, only
distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except
that UNION ALL selects all values.
The difference between Union and Union all is that
Union all will not eliminate duplicate rows, instead it just pulls all rows
from all tables fitting your query specifics and combines them into a table. ( Read More Here)
What is B‐Tree?
The database server uses a B‐tree structure to organize index information. B‐Tree generally has following types of index pages or
nodes:
root node: A root node contains node pointers to
branch nodes which can be only one.
branch nodes: A branch node contains pointers to leaf
nodes or other branch nodes which can be two or more.
leaf nodes A leaf node contains index items and
horizontal pointers to other leaf nodes which can be many.
No comments:
Post a Comment