SQL Server, DBA interview questions
- What is normalization? Explain different levels of normalization?
- Check out the article Q100139 from Microsoft knowledge base and of
course, there’s much more information available in the net. It’ll be a
good idea to get a hold of any RDBMS fundamentals text book, especially
the one by C. J. Date. Most of the times, it will be okay if you can
explain till third normal form.
- Check out the article Q100139 from Microsoft knowledge base and of
course, there’s much more information available in the net. It’ll be a
good idea to get a hold of any RDBMS fundamentals text book, especially
the one by C. J. Date. Most of the times, it will be okay if you can
explain till third normal form.
- What is denormalization and when would you go for it?
- As the name indicates, denormalization is the reverse process of
normalization. It’s the controlled introduction of redundancy in to the
database design. It helps improve the query performance as the number
of joins could be reduced.
- As the name indicates, denormalization is the reverse process of
normalization. It’s the controlled introduction of redundancy in to the
database design. It helps improve the query performance as the number
of joins could be reduced.
- How do you 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. It will be a good idea to read up a database designing
fundamentals text book.
- 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. It will be a good idea to read up a database designing
fundamentals text book.
- What’s the difference between a primary key and a unique key?
- Both primary key and unique enforce 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.
- Both primary key and unique enforce 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 are user defined datatypes and when you should go for them?
- User defined datatypes let you extend the base SQL Server datatypes by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8). In this case you could create a user defined datatype called Flight_num_type of varchar(8) and use it across all your tables. See sp_addtype, sp_droptype in books online.
- What is bit datatype and what’s the information that can be stored inside a bit column?
- Bit datatype is used to store boolean information like 1 or 0 (true
or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or
0 and there was no support for NULL. But from SQL Server 7.0 onwards,
bit datatype can represent a third state, which is NULL.
- Bit datatype is used to store boolean information like 1 or 0 (true
or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or
0 and there was no support for NULL. But from SQL Server 7.0 onwards,
bit datatype can represent a third state, which is NULL.
- Define candidate key, alternate key, composite key.
- A candidate key is one that can identify each row of a table
uniquely. Generally a candidate key becomes the primary key of the
table. If the table has more than one candidate key, one of them will
become the primary key, and the rest are called alternate keys. A key
formed by combining at least two or more columns is called composite
key.
- A candidate key is one that can identify each row of a table
uniquely. Generally a candidate key becomes the primary key of the
table. If the table has more than one candidate key, one of them will
become the primary key, and the rest are called alternate keys. A key
formed by combining at least two or more columns is called composite
key.
- What are defaults? Is there a column to which a default can’t be bound?
- A default is a value that will be used by a column, if no value is
supplied to that column while inserting data. IDENTITY columns and
timestamp columns can’t have defaults bound to them. See CREATE DEFAULT
in books online.
- A default is a value that will be used by a column, if no value is
supplied to that column while inserting data. IDENTITY columns and
timestamp columns can’t have defaults bound to them. See CREATE DEFAULT
in books online.
- What is a transaction and what are ACID properties?
- A transaction is a logical unit of work in which, all the steps
must be performed or none. ACID stands for Atomicity, Consistency,
Isolation, Durability. These are the properties of a transaction. For
more information and explanation of these properties, see SQL Server
books online or any RDBMS fundamentals text book. Explain different
isolation levels An isolation level determines the degree of isolation
of data between concurrent transactions. The default SQL Server
isolation level is Read Committed. Here are the other isolation levels
(in the ascending order of isolation): Read Uncommitted, Read
Committed, Repeatable Read, Serializable. See SQL Server books online
for an explanation of the isolation levels. Be sure to read about SET
TRANSACTION ISOLATION LEVEL, which lets you customize the isolation
level at the connection level. Read Committed - A transaction operating
at the Read Committed level cannot see changes made by other
transactions until those transactions are committed. At this level of
isolation, dirty reads are not possible but nonrepeatable reads and
phantoms are possible. Read Uncommitted - A transaction operating at
the Read Uncommitted level can see uncommitted changes made by other
transactions. At this level of isolation, dirty reads, nonrepeatable
reads, and phantoms are all possible. Repeatable Read - A transaction
operating at the Repeatable Read level is guaranteed not to see any
changes made by other transactions in values it has already read. At
this level of isolation, dirty reads and nonrepeatable reads are not
possible but phantoms are possible. Serializable - A transaction
operating at the Serializable level guarantees that all concurrent
transactions interact only in ways that produce the same effect as if
each transaction were entirely executed one after the other. At this
isolation level, dirty reads, nonrepeatable reads, and phantoms are not
possible.
- A transaction is a logical unit of work in which, all the steps
must be performed or none. ACID stands for Atomicity, Consistency,
Isolation, Durability. These are the properties of a transaction. For
more information and explanation of these properties, see SQL Server
books online or any RDBMS fundamentals text book. Explain different
isolation levels An isolation level determines the degree of isolation
of data between concurrent transactions. The default SQL Server
isolation level is Read Committed. Here are the other isolation levels
(in the ascending order of isolation): Read Uncommitted, Read
Committed, Repeatable Read, Serializable. See SQL Server books online
for an explanation of the isolation levels. Be sure to read about SET
TRANSACTION ISOLATION LEVEL, which lets you customize the isolation
level at the connection level. Read Committed - A transaction operating
at the Read Committed level cannot see changes made by other
transactions until those transactions are committed. At this level of
isolation, dirty reads are not possible but nonrepeatable reads and
phantoms are possible. Read Uncommitted - A transaction operating at
the Read Uncommitted level can see uncommitted changes made by other
transactions. At this level of isolation, dirty reads, nonrepeatable
reads, and phantoms are all possible. Repeatable Read - A transaction
operating at the Repeatable Read level is guaranteed not to see any
changes made by other transactions in values it has already read. At
this level of isolation, dirty reads and nonrepeatable reads are not
possible but phantoms are possible. Serializable - A transaction
operating at the Serializable level guarantees that all concurrent
transactions interact only in ways that produce the same effect as if
each transaction were entirely executed one after the other. At this
isolation level, dirty reads, nonrepeatable reads, and phantoms are not
possible.
- CREATE INDEX myIndex ON myTable(myColumn)What type of Index will get created after executing the above statement?
- Non-clustered index. Important thing to note: By default a
clustered index gets created on the primary key, unless specified
otherwise.
- Non-clustered index. Important thing to note: By default a
clustered index gets created on the primary key, unless specified
otherwise.
- What’s the maximum size of a row?
- 8060 bytes. Don’t be surprised with questions like ‘what is the
maximum number of columns per table’. 1024 columns per table. Check out
SQL Server books online for the page titled: "Maximum Capacity
Specifications". Explain Active/Active and Active/Passive cluster
configurations Hopefully you have experience setting up cluster
servers. But if you don’t, at least be familiar with the way clustering
works and the two clusterning configurations Active/Active and
Active/Passive. SQL Server books online has enough information on this
topic and there is a good white paper available on Microsoft site.
Explain the architecture of SQL Server This is a very important
question and you better be able to answer it if consider yourself a
DBA. SQL Server books online is the best place to read about SQL Server
architecture. Read up the chapter dedicated to SQL Server Architecture.
- 8060 bytes. Don’t be surprised with questions like ‘what is the
maximum number of columns per table’. 1024 columns per table. Check out
SQL Server books online for the page titled: "Maximum Capacity
Specifications". Explain Active/Active and Active/Passive cluster
configurations Hopefully you have experience setting up cluster
servers. But if you don’t, at least be familiar with the way clustering
works and the two clusterning configurations Active/Active and
Active/Passive. SQL Server books online has enough information on this
topic and there is a good white paper available on Microsoft site.
Explain the architecture of SQL Server This is a very important
question and you better be able to answer it if consider yourself a
DBA. SQL Server books online is the best place to read about SQL Server
architecture. Read up the chapter dedicated to SQL Server Architecture.
- What is lock escalation?
- Lock escalation is the process of converting a lot of low level
locks (like row locks, page locks) into higher level locks (like table
locks). Every lock is a memory structure too many locks would mean,
more memory being occupied by locks. To prevent this from happening,
SQL Server escalates the many fine-grain locks to fewer coarse-grain
locks. Lock escalation threshold was definable in SQL Server 6.5, but
from SQL Server 7.0 onwards it’s dynamically managed by SQL Server.
- Lock escalation is the process of converting a lot of low level
locks (like row locks, page locks) into higher level locks (like table
locks). Every lock is a memory structure too many locks would mean,
more memory being occupied by locks. To prevent this from happening,
SQL Server escalates the many fine-grain locks to fewer coarse-grain
locks. Lock escalation threshold was definable in SQL Server 6.5, but
from SQL Server 7.0 onwards it’s dynamically managed by SQL Server.
- What’s the difference between DELETE TABLE and TRUNCATE TABLE commands?
- DELETE TABLE is a logged operation, so the deletion of each row
gets logged in the transaction log, which makes it slow. TRUNCATE TABLE
also deletes all the rows in a table, but it won’t log the deletion of
each row, instead it logs the deallocation of the data pages of the
table, which makes it faster. Of course, TRUNCATE TABLE can be rolled
back. TRUNCATE TABLE is functionally identical to DELETE statement with
no WHERE clause: both remove all rows in the table. But TRUNCATE TABLE
is faster and uses fewer system and transaction log resources than
DELETE. The DELETE statement removes rows one at a time and records an
entry in the transaction log for each deleted row. TRUNCATE TABLE
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 TABLE removes all rows from a table, but the
table structure and its columns, constraints, indexes and so on remain.
The counter used by an identity for new rows is reset to the seed for
the column. 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. You cannot use TRUNCATE TABLE on a table
referenced by a FOREIGN KEY constraint; instead, use DELETE statement
without a WHERE clause. Because TRUNCATE TABLE is not logged, it cannot
activate a trigger. TRUNCATE TABLE may not be used on tables
participating in an indexed view
- DELETE TABLE is a logged operation, so the deletion of each row
gets logged in the transaction log, which makes it slow. TRUNCATE TABLE
also deletes all the rows in a table, but it won’t log the deletion of
each row, instead it logs the deallocation of the data pages of the
table, which makes it faster. Of course, TRUNCATE TABLE can be rolled
back. TRUNCATE TABLE is functionally identical to DELETE statement with
no WHERE clause: both remove all rows in the table. But TRUNCATE TABLE
is faster and uses fewer system and transaction log resources than
DELETE. The DELETE statement removes rows one at a time and records an
entry in the transaction log for each deleted row. TRUNCATE TABLE
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 TABLE removes all rows from a table, but the
table structure and its columns, constraints, indexes and so on remain.
The counter used by an identity for new rows is reset to the seed for
the column. 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. You cannot use TRUNCATE TABLE on a table
referenced by a FOREIGN KEY constraint; instead, use DELETE statement
without a WHERE clause. Because TRUNCATE TABLE is not logged, it cannot
activate a trigger. TRUNCATE TABLE may not be used on tables
participating in an indexed view
- Explain the storage models of OLAP
- Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
- Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
- What are the new features introduced in SQL Server 2000 (or the
latest release of SQL Server at the time of your interview)? What
changed between the previous version of SQL Server and the current
version?
- This question is generally asked to see how current is your
knowledge. Generally there is a section in the beginning of the books
online titled "What’s New", which has all such information. Of course,
reading just that is not enough, you should have tried those things to
better answer the questions. Also check out the section titled
"Backward Compatibility" in books online which talks about the changes
that have taken place in the new version.
- This question is generally asked to see how current is your
knowledge. Generally there is a section in the beginning of the books
online titled "What’s New", which has all such information. Of course,
reading just that is not enough, you should have tried those things to
better answer the questions. Also check out the section titled
"Backward Compatibility" in books online which talks about the changes
that have taken place in the new version.
- What are constraints? Explain different types of constraints.
- Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or
defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY,
FOREIGN KEY. For an explanation of these constraints see books online
for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
- Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or
defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY,
FOREIGN KEY. For an explanation of these constraints see books online
for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
- What is an index? What are the types of indexes? How many clustered
indexes can be created on a table? I create a separate index on each
column of a table. What are the advantages and disadvantages of this
approach?
- Indexes in SQL Server are similar to the indexes in books. They
help SQL Server retrieve the data quicker. Indexes are of two types.
Clustered indexes and non-clustered indexes. When you create a
clustered index on a table, all the rows in the table are stored in the
order of the clustered index key. So, there can be only one clustered
index per table. Non-clustered indexes have their own storage separate
from the table data storage. Non-clustered indexes are stored as B-tree
structures (so do clustered indexes), with the leaf level nodes having
the index key and it’s row locater. The row located could be the RID or
the Clustered index key, depending up on the absence or presence of
clustered index on the table. If you create an index on each column of
a table, it improves the query performance, as the query optimizer can
choose from all the existing indexes to come up with an efficient
execution plan. At the same t ime, data modification operations (such
as INSERT, UPDATE, DELETE) will become slow, as every time data changes
in the table, all the indexes need to be updated. Another disadvantage
is that, indexes need disk space, the more indexes you have, more disk
space is used.
- Indexes in SQL Server are similar to the indexes in books. They
help SQL Server retrieve the data quicker. Indexes are of two types.
Clustered indexes and non-clustered indexes. When you create a
clustered index on a table, all the rows in the table are stored in the
order of the clustered index key. So, there can be only one clustered
index per table. Non-clustered indexes have their own storage separate
from the table data storage. Non-clustered indexes are stored as B-tree
structures (so do clustered indexes), with the leaf level nodes having
the index key and it’s row locater. The row located could be the RID or
the Clustered index key, depending up on the absence or presence of
clustered index on the table. If you create an index on each column of
a table, it improves the query performance, as the query optimizer can
choose from all the existing indexes to come up with an efficient
execution plan. At the same t ime, data modification operations (such
as INSERT, UPDATE, DELETE) will become slow, as every time data changes
in the table, all the indexes need to be updated. Another disadvantage
is that, indexes need disk space, the more indexes you have, more disk
space is used.
- What is RAID and what are different types of RAID configurations?
- RAID stands for Redundant Array of Inexpensive Disks, used to
provide fault tolerance to database servers. There are six RAID levels
0 through 5 offering different levels of performance, fault tolerance.
MSDN has some information about RAID levels and for detailed
information, check out the RAID advisory board’s homepage
- RAID stands for Redundant Array of Inexpensive Disks, used to
provide fault tolerance to database servers. There are six RAID levels
0 through 5 offering different levels of performance, fault tolerance.
MSDN has some information about RAID levels and for detailed
information, check out the RAID advisory board’s homepage
- What are the steps you will take to improve performance of a poor performing query?
- This is a very open ended question and there could be a lot of
reasons behind the poor performance of a query. But some general issues
that you could talk about would be: No indexes, table scans, missing or
out of date statistics, blocking, excess recompilations of stored
procedures, procedures and triggers without SET NOCOUNT ON, poorly
written query with unnecessarily complicated joins, too much
normalization, excess usage of cursors and temporary tables. Some of
the tools/ways that help you troubleshooting performance problems are:
SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL
Server Profiler, Windows NT /2000 Performance monitor, Graphical
execution plan in Query Analyzer. Download the white paper on
performance tuning SQL Server from Microsoft web site. Don’t forget to
check out sql-server-performance.com
- This is a very open ended question and there could be a lot of
reasons behind the poor performance of a query. But some general issues
that you could talk about would be: No indexes, table scans, missing or
out of date statistics, blocking, excess recompilations of stored
procedures, procedures and triggers without SET NOCOUNT ON, poorly
written query with unnecessarily complicated joins, too much
normalization, excess usage of cursors and temporary tables. Some of
the tools/ways that help you troubleshooting performance problems are:
SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL
Server Profiler, Windows NT /2000 Performance monitor, Graphical
execution plan in Query Analyzer. Download the white paper on
performance tuning SQL Server from Microsoft web site. Don’t forget to
check out sql-server-performance.com
- What are the steps you will take, if you are tasked with securing an SQL Server?
- Again this is another open ended question. Here are some things you
could talk about: Preferring NT authentication, using server, databse
and application roles to control access to the data, securing the
physical database files using NTFS permissions, using an unguessable SA
password, restricting physical access to the SQL Server, renaming the
Administrator account on the SQL Server computer, disabling the Guest
account, enabling auditing, using multiprotocol encryption, setting up
SSL, setting up firewalls, isolating SQL Server from the web server
etc. Read the white paper on SQL Server security from Microsoft
website. Also check out My SQL Server security best practices
- Again this is another open ended question. Here are some things you
could talk about: Preferring NT authentication, using server, databse
and application roles to control access to the data, securing the
physical database files using NTFS permissions, using an unguessable SA
password, restricting physical access to the SQL Server, renaming the
Administrator account on the SQL Server computer, disabling the Guest
account, enabling auditing, using multiprotocol encryption, setting up
SSL, setting up firewalls, isolating SQL Server from the web server
etc. Read the white paper on SQL Server security from Microsoft
website. Also check out My SQL Server security best practices
- What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
- Deadlock is a situation when two processes, each having a lock on
one piece of data, attempt to acquire a lock on the other’s piece. Each
process would wait indefinitely for the other to release the lock,
unless one of the user processes is terminated. SQL Server detects
deadlocks and terminates one user’s process. A livelock is one, where a
request for an exclusive lock is repeatedly denied because a series of
overlapping shared locks keeps interfering. SQL Server detects the
situation after four denials and refuses further shared locks. A
livelock also occurs when read transactions monopolize a table or page,
forcing a write transaction to wait indefinitely. Check out SET
DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books
online.
- Deadlock is a situation when two processes, each having a lock on
one piece of data, attempt to acquire a lock on the other’s piece. Each
process would wait indefinitely for the other to release the lock,
unless one of the user processes is terminated. SQL Server detects
deadlocks and terminates one user’s process. A livelock is one, where a
request for an exclusive lock is repeatedly denied because a series of
overlapping shared locks keeps interfering. SQL Server detects the
situation after four denials and refuses further shared locks. A
livelock also occurs when read transactions monopolize a table or page,
forcing a write transaction to wait indefinitely. Check out SET
DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books
online.
- What is blocking and how would you troubleshoot it?
- Blocking happens when one connection from an application holds a
lock and a second connection requires a conflicting lock type. This
forces the second connection to wait, blocked on the first. Read up the
following topics in SQL Server books online: Understanding and avoiding
blocking, Coding efficient transactions. Explain CREATE DATABASE syntax
Many of us are used to creating databases from the Enterprise Manager
or by just issuing the command: CREATE DATABAE MyDB.
- Blocking happens when one connection from an application holds a
lock and a second connection requires a conflicting lock type. This
forces the second connection to wait, blocked on the first. Read up the
following topics in SQL Server books online: Understanding and avoiding
blocking, Coding efficient transactions. Explain CREATE DATABASE syntax
Many of us are used to creating databases from the Enterprise Manager
or by just issuing the command: CREATE DATABAE MyDB.
- But what if you have to create a database with two filegroups, one
on drive C and the other on drive D with log on drive E with an initial
size of 600 MB and with a growth factor of 15%?
- That’s why being a DBA you should be familiar with the CREATE
DATABASE syntax. Check out SQL Server books online for more
information.
- That’s why being a DBA you should be familiar with the CREATE
DATABASE syntax. Check out SQL Server books online for more
information.
- How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
- SQL Server can be started from command line, using the
SQLSERVR.EXE. This EXE has some very important parameters with which a
DBA should be familiar with. -m is used for starting SQL Server in
single user mode and -f is used to start the SQL Server in minimal
configuration mode. Check out SQL Server books online for more
parameters and their explanations.
- SQL Server can be started from command line, using the
SQLSERVR.EXE. This EXE has some very important parameters with which a
DBA should be familiar with. -m is used for starting SQL Server in
single user mode and -f is used to start the SQL Server in minimal
configuration mode. Check out SQL Server books online for more
parameters and their explanations.
- As a part of your job, what are the DBCC commands that you commonly use for database maintenance?
- DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC,
DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there
are a whole load of DBCC commands which are very useful for DBAs. Check
out SQL Server books online for more information.
- DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC,
DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there
are a whole load of DBCC commands which are very useful for DBAs. Check
out SQL Server books online for more information.
- What are statistics, under what circumstances they go out of date, how do you update them?
- Statistics determine the selectivity of the indexes. If an indexed
column has unique values then the selectivity of that index is more, as
opposed to an index with non-unique values. Query optimizer uses these
indexes in determining whether to choose an index or not while
executing a query. Some situations under which you should update
statistics: 1) If there is significant change in the key values in the
index 2) If a large amount of data in an indexed column has been added,
changed, or removed (that is, if the distribution of key values has
changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated 3) Database is upgraded from a previous
version. Look up SQL Server books online for the following commands:
UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS,
DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats
- Statistics determine the selectivity of the indexes. If an indexed
column has unique values then the selectivity of that index is more, as
opposed to an index with non-unique values. Query optimizer uses these
indexes in determining whether to choose an index or not while
executing a query. Some situations under which you should update
statistics: 1) If there is significant change in the key values in the
index 2) If a large amount of data in an indexed column has been added,
changed, or removed (that is, if the distribution of key values has
changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated 3) Database is upgraded from a previous
version. Look up SQL Server books online for the following commands:
UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS,
DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats
- What are the different ways of moving data/databases between servers and databases in SQL Server?
- There are lots of options available, you have to choose your option
depending upon your requirements. Some of the options you have are:
BACKUP/RESTORE, dettaching and attaching databases, replication, DTS,
BCP, logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts
to generate data.
- There are lots of options available, you have to choose your option
depending upon your requirements. Some of the options you have are:
BACKUP/RESTORE, dettaching and attaching databases, replication, DTS,
BCP, logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts
to generate data.
- Explain different types of BACKUPs avaialabe in SQL Server? Given a
particular scenario, how would you go about choosing a backup plan?
- Types of backups you can create in SQL Sever 7.0+ are Full database
backup, differential database backup, transaction log backup, filegroup
backup. Check out the BACKUP and RESTORE commands in SQL Server books
online. Be prepared to write the commands in your interview. Books
online also has information on detailed backup/restore architecture and
when one should go for a particular kind of backup.
- Types of backups you can create in SQL Sever 7.0+ are Full database
backup, differential database backup, transaction log backup, filegroup
backup. Check out the BACKUP and RESTORE commands in SQL Server books
online. Be prepared to write the commands in your interview. Books
online also has information on detailed backup/restore architecture and
when one should go for a particular kind of backup.
- What is database replication? What are the different types of replication you can set up in SQL Server?
- Replication is the process of copying/moving data between databases
on the same or different servers. SQL Server supports the following
types of replication scenarios: � Snapshot replication � Transactional
replication (with immediate updating subscribers, with queued updating
subscribers) � Merge replication See SQL Server books online for
indepth coverage on replication. Be prepared to explain how different
replication agents function, what are the main system tables used in
replication etc.
- Replication is the process of copying/moving data between databases
on the same or different servers. SQL Server supports the following
types of replication scenarios: � Snapshot replication � Transactional
replication (with immediate updating subscribers, with queued updating
subscribers) � Merge replication See SQL Server books online for
indepth coverage on replication. Be prepared to explain how different
replication agents function, what are the main system tables used in
replication etc.
- How to determine the service pack currently installed on SQL Server?
- The global variable @@Version stores the build number of the
sqlservr.exe, which is used to determine the service pack installed. To
know more about this process visit SQL Server service packs and
versions.
- The global variable @@Version stores the build number of the
sqlservr.exe, which is used to determine the service pack installed. To
know more about this process visit SQL Server service packs and
versions.
- What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
- Cursors allow row-by-row processing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria: Salary between 30000 and 40000 — 5000 hike Salary between 40000 and 55000 — 7000 hike Salary between 55000 and 65000 — 9000 hike. In this situation many developers tend to use a cursor, determine each employee’s salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:
- UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END
- Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don’t have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the ‘My code library’ section of my site or search for WHILE. Write down the general syntax for a SELECT statements covering all the options. Here’s the basic syntax: (Also checkout SELECT in books online for advanced syntax).
- SELECT select_list [INTO
new_table_] FROM table_source [WHERE search_condition] [GROUP BY
group_by_expression] [HAVING search_condition] [ORDER BY
order_expression [ASC | DESC] ]
- What is a join and explain different types of joins.
- Joins are used in queries to explain how different tables are
related. Joins also let you select data from a table depending upon
data from another table. Types of joins: INNER JOINs, OUTER JOINs,
CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS,
RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages
from books online titled: "Join Fundamentals" and "Using Joins".
- Joins are used in queries to explain how different tables are
related. Joins also let you select data from a table depending upon
data from another table. Types of joins: INNER JOINs, OUTER JOINs,
CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS,
RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages
from books online titled: "Join Fundamentals" and "Using Joins".
- Can you have a nested transaction?
- Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
- Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
- What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
- An extended stored procedure is a function within a DLL (written in
a programming language like C, C++ using Open Data Services (ODS) API)
that can be called from T-SQL, just the way we call normal stored
procedures using the EXEC statement. See books online to learn how to
create extended stored procedures and how to add them to SQL Server.
Yes, you can instantiate a COM (written in languages like VB, VC++)
object from T-SQL by using sp_OACreate stored procedure. Also see books
online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty,
sp_OADestroy. For an example of creating a COM object in VB and calling
it from T-SQL, see ‘My code library’ section of this site.
- An extended stored procedure is a function within a DLL (written in
a programming language like C, C++ using Open Data Services (ODS) API)
that can be called from T-SQL, just the way we call normal stored
procedures using the EXEC statement. See books online to learn how to
create extended stored procedures and how to add them to SQL Server.
Yes, you can instantiate a COM (written in languages like VB, VC++)
object from T-SQL by using sp_OACreate stored procedure. Also see books
online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty,
sp_OADestroy. For an example of creating a COM object in VB and calling
it from T-SQL, see ‘My code library’ section of this site.
- What is the system function to get the current user’s user id?
- USER_ID(). Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(),
HOST_NAME().
- USER_ID(). Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(),
HOST_NAME().
- What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?
- Triggers are special kind of stored procedures that get executed
automatically when an INSERT, UPDATE or DELETE operation takes place on
a table. In SQL Server 6.5 you could define only 3 triggers per table,
one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0
onwards, this restriction is gone, and you could create multiple
triggers per each action. But in 7.0 there’s no way to control the
order in which the triggers fire. In SQL Server 2000 you could specify
which trigger fires first or fires last using sp_settriggerorder.
Triggers can’t be invoked on demand. They get triggered only when an
associated action (INSERT, UPDATE, DELETE) happens on the table on
which they are defined. Triggers are generally used to implement
business rules, auditing. Triggers can also be used to extend the
referential integrity checks, but wherever possible, use constraints
for this purpose, instead of triggers, as constraints are much faster.
Till SQL Server 7.0, triggers fire only after the data modification
operation happens. So in a way, they are called post triggers. But in
SQL Server 2000 you could create pre triggers also. Search SQL Server
2000 books online for INSTEAD OF triggers. Also check out books online
for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()
- Triggers are special kind of stored procedures that get executed
automatically when an INSERT, UPDATE or DELETE operation takes place on
a table. In SQL Server 6.5 you could define only 3 triggers per table,
one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0
onwards, this restriction is gone, and you could create multiple
triggers per each action. But in 7.0 there’s no way to control the
order in which the triggers fire. In SQL Server 2000 you could specify
which trigger fires first or fires last using sp_settriggerorder.
Triggers can’t be invoked on demand. They get triggered only when an
associated action (INSERT, UPDATE, DELETE) happens on the table on
which they are defined. Triggers are generally used to implement
business rules, auditing. Triggers can also be used to extend the
referential integrity checks, but wherever possible, use constraints
for this purpose, instead of triggers, as constraints are much faster.
Till SQL Server 7.0, triggers fire only after the data modification
operation happens. So in a way, they are called post triggers. But in
SQL Server 2000 you could create pre triggers also. Search SQL Server
2000 books online for INSTEAD OF triggers. Also check out books online
for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()
- There is a trigger defined for INSERT operations on a table, in an
OLTP system. The trigger is written to instantiate a COM object and
pass the newly insterted rows to it for some custom processing. What do
you think of this implementation? Can this be implemented better?
- Instantiating COM objects is a time consuming process and since you
are doing it from within a trigger, it slows down the data insertion
process. Same is the case with sending emails from triggers. This
scenario can be better implemented by logging all the necessary data
into a separate table, and have a job which periodically checks this
table and does the needful.
- Instantiating COM objects is a time consuming process and since you
are doing it from within a trigger, it slows down the data insertion
process. Same is the case with sending emails from triggers. This
scenario can be better implemented by logging all the necessary data
into a separate table, and have a job which periodically checks this
table and does the needful.
- What is a self join? Explain it with an example.
- Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.
- CREATE TABLE emp ( empid int, mgrid int, empname char(10) )
- INSERT emp SELECT 1,2,’Vyas’ INSERT emp SELECT 2,3,’Mohan’ INSERT emp SELECT 3,NULL,’Shobha’ INSERT emp SELECT 4,2,’Shridhar’ INSERT emp SELECT 5,2,’Sourabh’
- SELECT t1.empname [Employee], t2.empname [Manager] FROM emp t1, emp t2 WHERE t1.mgrid = t2.empid Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)
- SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid
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