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
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. (Read More Here)
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. (Read
More Here)
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.
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.
What
are the difference between clustered and a non-clustered index?
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.
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 transaction 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.
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.
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.
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
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
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
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
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.
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.
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.
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 nin 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.DepartmentID
GO
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.DepartmentID
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.
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.
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.
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.
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).
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 thesys.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.
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.
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
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.
i.
“Detach Database” from one
server and “Attach Database” to another server.
ii.
Manually script all the objects
using SSMS and run the script on new server.
iii.
Use Wizard of SSMS.
How
to copy data from one table to another table?
There are multiple ways to do
this.
1) 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.
2)
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.
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.
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.
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.
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.
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.
Returns the rank of rows within the partition of a result set, without any gaps in the ranking.
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.
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 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.
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.
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