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Rodney Vinyard - .NET & SQL Developer When all is said and done, more will be said than done
 

Normalization

1.      A table should have an identifier.

  1. A table should store only data for a single type of entity.
  2. A table should not have repeating values or columns

4.      A table should avoid nullable columns

 

Choosing a SQL Server Monitoring Tool

SQL Profiler

Enables you to monitor server and database activity (for example, number of deadlocks, fatal errors, tracing stored procedures and Transact-SQL statements, or login activity). You can capture SQL Profiler data to a SQL Server table or a file for later analysis, and also replay the events captured on SQL Server, step by step, to see exactly what happened. SQL Profiler tracks engine process events, such as the start of a batch or a transaction.

Windows Server System Monitor

Enables you to monitor server performance and activity using predefined objects and counters or user-defined counters to monitor events. System Monitor (Performance Monitor in Microsoft Windows NT® 4.0) collects counts rather than data about the events (for example, memory usage, number of active transactions, number of blocked locks, or CPU activity). You can set thresholds on specific counters to generate alerts that notify operators. System Monitor primarily tracks resource usage, such as the number of buffer manager page requests in use.

System Monitor works only on Microsoft Windows® 2000 and can monitor (remotely or locally) an instance of SQL Server on Windows NT 4.0 or Windows 2000 only.

Current activity window (SQL Server Enterprise Manager)

Graphically displays information about processes running currently on an instance of SQL Server, blocked processes, locks, and user activity. This is useful for ad hoc views of current activity.

Error Logs

Contain additional information about events in SQL Server than is available elsewhere. You can use the information in the error log to troubleshoot SQL Server-related problems. The Windows application event log provides an overall picture of events occurring on the Windows NT 4.0 and Windows 2000 system as a whole, as well as events in SQL Server, SQL Server Agent, and full-text search.

sp_who

Reports snapshot information about current SQL Server users and processes, including the currently executing statement and whether the statement is blocked. This is a Transact-SQL alternative to viewing user activity in the current activity window in SQL Server Enterprise Manager.

sp_lock

Reports snapshot information about locks, including the object ID, index ID, type of lock, and type or resource to which the lock applies. This is a Transact-SQL alternative to viewing lock activity in the current activity window in SQL Server Enterprise Manager.

sp_spaceused

Displays an estimate of the current amount of disk space used by a table (or a whole database). This is a Transact-SQL alternative to viewing database usage in SQL Server Enterprise Manager.

sp_monitor

Displays statistics, including CPU usage, I/O usage, and the amount of time idle since sp_monitor was last executed.

DBCC statements

Enables you to check performance statistics and the logical and physical consistency of a database. For more information, see DBCC.

Built-in functions

Display snapshot statistics about SQL Server activity since the server was started; these statistics are stored in predefined SQL Server counters. For example, @@CPU_BUSY contains the amount of time the CPU has been executing SQL Server code; @@CONNECTIONS contains the number of SQL Server connections or attempted connections; and @@PACKET_ERRORS contains the number of network packets occurring on SQL Server connections. For more information, see Functions.

SQL Profiler stored procedures and functions

Use Transact-SQL stored procedures to gather SQL Profiler statistics. For more information, see System Stored Procedures.

Trace flags

Display information about a specific activity within the server and are used to diagnose problems or performance issues (for example, deadlock chains). For more information, see Trace Flags.

Simple Network Management Protocol (SNMP)

Simple Network Management Protocol (SNMP) is an application protocol that offers network management services. Using SNMP, you can monitor an instance of SQL Server across different platforms (for example, Windows NT 4.0, Windows 98, and UNIX). With SQL Server and the Microsoft SQL Server Management Information Base (MSSQL-MIB), you can use SNMP applications to monitor the status of SQL Server installations. You can monitor performance information, access databases, and view server and database configuration parameters.

The choice of a monitoring tool depends on the type of events and activity to be monitored.



Event or activity


SQL Profiler


System Monitor

Current activity window


Transact-SQL


Error logs

Trend analysis

Yes

Yes

 

 

 

Replaying captured events

Yes

 

 

 

 

Ad hoc monitoring

Yes

 

Yes

Yes

Yes

Generating alerts

 

Yes

 

 

 

Graphical interface

Yes

Yes

Yes

 

Yes

Using within custom application

Yes 1

 

 

Yes

 

 

1 Using SQL Profiler system stored procedures.

The key difference between the two main monitoring tools, SQL Profiler and System Monitor, is that SQL Profiler monitors engine events while System Monitor monitors resource usage associated with server processes. For example, SQL Profiler can be used to monitor deadlocks events, including the users and objects involved in the deadlock. System Monitor can be used to monitor the total number of deadlocks occurring in a database or on a specific object.

Windows NT 4.0 and Windows 2000 also provides these monitoring tools:

  • Task Manager

Shows a synopsis of the processes and applications running on the system.

  • Network Monitor Agent

Assists in monitoring network traffic.

For more information about Windows NT 4.0 or Windows 2000 tools, see the Windows NT 4.0 or Windows 2000 documentation.

Concurrency Problems

If locking is not available and several users access a database concurrently, problems may occur if their transactions use the same data at the same time. Concurrency problems include:

Lost Updates

Lost updates occur when two or more transactions select the same row and then update the row based on the value originally selected. Each transaction is unaware of other transactions. The last update overwrites updates made by the other transactions, which results in lost data.

For example, two editors make an electronic copy of the same document. Each editor changes the copy independently and then saves the changed copy, thereby overwriting the original document. The editor who saves the changed copy last overwrites changes made by the first editor. This problem could be avoided if the second editor could not make changes until the first editor had finished.

Uncommitted Dependency (Dirty Read)

Uncommitted dependency occurs when a second transaction selects a row that is being updated by another transaction. The second transaction is reading data that has not been committed yet and may be changed by the transaction updating the row.

For example, an editor is making changes to an electronic document. During the changes, a second editor takes a copy of the document that includes all the changes made so far, and distributes the document to the intended audience. The first editor then decides the changes made so far are wrong and removes the edits and saves the document. The distributed document contains edits that no longer exist, and should be treated as if they never existed. This problem could be avoided if no one could read the changed document until the first editor determined that the changes were final.

Inconsistent Analysis (Nonrepeatable Read)

Inconsistent analysis occurs when a second transaction accesses the same row several times and reads different data each time. Inconsistent analysis is similar to uncommitted dependency in that another transaction is changing the data that a second transaction is reading. However, in inconsistent analysis, the data read by the second transaction was committed by the transaction that made the change. Also, inconsistent analysis involves multiple reads (two or more) of the same row and each time the information is changed by another transaction; thus, the term nonrepeatable read.

For example, an editor reads the same document twice, but between each reading, the writer rewrites the document. When the editor reads the document for the second time, it has changed. The original read was not repeatable. This problem could be avoided if the editor could read the document only after the writer has finished writing it.

Phantom Reads

Phantom reads occur when an insert or delete action is performed against a row that belongs to a range of rows being read by a transaction. The transaction's first read of the range of rows shows a row that no longer exists in the second or succeeding read, as a result of a deletion by a different transaction. Similarly, as the result of an insert by a different transaction, the transaction's second or succeeding read shows a row that did not exist in the original read.

For example, an editor makes changes to a document submitted by a writer, but when the changes are incorporated into the master copy of the document by the production department, they find that new unedited material has been added to the document by the author. This problem could be avoided if no one could add new material to the document until the editor and production department finish working with the original document.

 

SQL-92 defines the following isolation levels, all of which are supported by SQL Server:

  • Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
  • Read committed (SQL Server default level).
  • Repeatable read.
  • Serializable (the highest level, where transactions are completely isolated from one another).

If transactions are run at an isolation level of serializable, any concurrent overlapping transactions are guaranteed to be serializable.

These isolation levels allow different types of behavior.

Isolation level

Dirty read

Nonrepeatable read

Phantom

Read uncommitted

Yes

Yes

Yes

Read committed

No

Yes

Yes

Repeatable read

No

No

Yes

Serializable

No

No

No

 

Query Tuning Recommendations

Some queries are inherently resource intensive. This is related to fundamental database and index issues. These queries are not inefficient, because the query optimizer will implement the queries in the most efficient fashion possible. However, they are resource intensive, and the set-oriented nature of Transact-SQL can make them appear inefficient. No degree of query optimizer intelligence can eliminate the inherent resource cost of these constructs. They are intrinsically costly when compared to a less complex query. Although Microsoft® SQL Server™ 2000 uses the most optimal access plan, this is limited by what is fundamentally possible. For example, the following types of queries can be resource intensive:

  • Queries returning large result sets
  • Highly nonunique WHERE clauses

However, recommendations for tuning queries and improving query performance include:

    • If the query uses cursors, determine if the cursor query could be written more efficiently using either a more efficient cursor type, such as fast forward-only, or a single query. Single queries typically outperform cursor operations. Because a set of cursor statements is typically an outer loop operation, in which each row in the outer loop is processed once using an inner statement, consider using either a GROUP BY or CASE statement or a subquery instead.
    • If an application uses a loop, consider putting the loop inside the query. Often an application will contain a loop that contains a parameterized query, which is executed many times and requires a network round trip between the computer running the application and SQL Server. Instead, create a single, more complex query using a temporary table. Only one network round trip is necessary, and the query optimizer can better optimize the single query.
    • Do not use multiple aliases for a single table in the same query to simulate index intersection. This is no longer necessary because SQL Server automatically considers index intersection and can make use of multiple indexes on the same table in the same query. For example, given the sample query:
·                      SELECT * FROM lineitem 
·                      WHERE partkey BETWEEN 17000 AND 17100 AND
·                            shipdate BETWEEN '1/1/1994' AND '1/31/1994"

SQL Server can exploit indexes on both the partkey and shipdate columns, and then perform a hash match between the two subsets to obtain the index intersection.

    • Make use of query hints only if necessary. Queries using hints executed against earlier versions of SQL Server should be tested without the hints specified. The hints can prevent the query optimizer from choosing a better execution plan. For more information, see SELECT.
  • Make use of the query governor configuration option and setting. The query governor configuration option can be used to prevent long-running queries from executing, thus preventing system resources from being consumed. By default, the query governor configuration option allows all queries to execute, no matter how long they take. However, the query governor can be set to the maximum number of seconds that all queries for all connections, or just the queries for a specific connection, are allowed to execute. Because the query governor is based on estimated query cost, rather than actual elapsed time, it does not have any run-time overhead. It also stops long-running queries before they start, rather than running them until some predefined limit is hit. For more information, see query governor cost limit Option and SET QUERY_GOVERNOR_COST_LIMIT.

Optimizing Application Performance Using Efficient Data Retrieval

Some applications cannot buffer all the data they request from the server. For example, an application that queries a large table and allows the user to specify the selection criteria may return no rows or millions of rows. The user is unlikely to want to see millions of rows. Instead, the user is more likely to reexecute the query with narrower selection criteria. In this case, fetching and buffering millions of rows only to have them thrown away by the user wastes time and resources.

For these applications, SQL Server offers server cursors that allow an application to fetch a small subset or block of rows from an arbitrarily large result set. If the user wants to see other records from the same result set, a server cursor allows the application to fetch any other block of rows from the result set, including the next n rows, the previous n rows, or n rows starting at a certain row number in the result set. SQL Server does the work to fulfill each block fetch request only as needed, and SQL Server does not normally hold locks between block fetches on server cursors.

Server cursors also allow an application to do a positioned update or delete of a fetched row without having to figure out the source table and primary key of the row. If the row data changes between the time it is fetched and the time the update is requested, SQL Server detects the problem and prevents a lost update.

However, the features of server cursors come at a cost. If all the results from a given query are going to be used in your application, a server cursor is always going to be more expensive than a default result set. A default result set always requires only one roundtrip between client and server, whereas each call to fetch a block of rows from a server cursor results in a roundtrip. Moreover, server cursors consume resources on the server, and there are restrictions on the SELECT statements that can be used with some types of cursor. For example, KEYSET cursors are restricted to using tables with unique indexes only, while KEYSET and STATIC cursors make heavy use of temporary storage at the server. For these reasons, only use server cursors when your application needs their features. If a particular task requests a single row by primary key, use a default result set. If another task requires an unpredictably large or updatable result set, use a server cursor and fetch rows in reasonably sized blocks (for example, one screen of rows at a time). Additionally, where possible, make use of Fast Forward-only cursors with auto-fetch. These cursors can be used to retrieve small result sets with only one roundtrip between the client and server, similar to a default result set. For more information, see Fast Forward-only Cursors.

Cursor types supported by SQL Server

FORWARD_ONLY - specifies that cursor can only fetch data sequentially from the first to the last row. FETCH NEXT is the only fetch option supported.

FAST_FORWARD - specifies that cursor will be FORWARD_ONLY and READ_ONLY cursor. The FAST_FORWARD cursors produce the least amount of overhead on SQL Server.

READ ONLY - specifies that cursor cannot be updated.Rules for Choosing a Cursor Type
STATIC - specifies that cursor will use a temporary copy of the data instead of base tables. This cursor does not allow modifications and modifications made to base tables are not reflected in the data returned by fetches made to this cursor.

KEYSET - specifies that cursor uses the set of keys that uniquely identify the cursor's rows (keyset), so that the membership and order of rows in the cursor are fixed when the cursor is opened. SQL Server uses a table in tempdb to store keyset. The KEYSET cursor allows updates nonkey values from being made through this cursor, but inserts made by other users are not visible. Updates nonkey values made by other users are visible as the owner scrolls around the cursor, but updates key values made by other users are not visible. If a row is deleted, an attempt to fetch the row returns an @@FETCH_STATUS of -2.

DYNAMIC - specifies that cursor reflects all data changes made to the base tables as you scroll around the cursor. FETCH ABSOLUTE option is not supported with DYNAMIC cursor.


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.

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.

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.

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.

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. 

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. 

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 DEFUALT in books online.

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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.

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.

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'. 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.

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.

Explain the storage models of OLAP

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.

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"

Whar 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 craete 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.

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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

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

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

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. Also check out the article Q169960 from Microsoft knowledge base.

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 craeting 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.

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 confuguration 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.

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

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.

Explian 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.

What is database replicaion? 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.

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.

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What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row prcessing 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 rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, 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".

Can you have a nested transaction?

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.

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().

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()

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.

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
 

Given an employee table, how would you find out the second highest salary?

SELECT MIN(SALARY) FROM EMP  WHERE
SALARY IN (SELECT DISTINCT TOP N SALARY FROM EMP ORDER BY SALARY DESC)

 

Posted on Tuesday, July 31, 2007 3:18 AM Interview Questions | Back to top

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