sql server interview questions
1. 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.
2. 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.
3. 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
4. What are different types of
Collation Sensitivity?
- Case sensitivity - A and a, B and b, etc.
- Accent sensitivity
- 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.
5. 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.
6. 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.
7. What is difference between DELETE
and 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.
Note: DELETE and TRUNCATE both can
be rolled back when surrounded by TRANSACTION if the current session is not
closed. If TRUNCATE is written in Query Editor surrounded by TRANSACTION and if
session is closed, it can not be rolled back but DELETE can be rolled back.
8. 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.
9. 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.
10. 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
11. 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.
12. What are the authentication
modes in SQL Server? How can it be changed?
Windows mode and Mixed Mode - SQL
and 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.
13. Which command using Query Analyzer will give you the
version of SQL server and operating system?
SELECT SERVERPROPERTY
('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY
('edition').
14. 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.
15. 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.
16. 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.
17. 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
18. 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.
19. 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.
20. 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.
21. 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. 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
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
32. 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.
33. 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.