SQL Server DBA Interview Questions

 Q.What are the different types of backups?

Answer: The SQL server offers 4 types of backups to suit the need of the administrator.

Complete backup :The complete back up is just zipping the content of the entire database in terms of the different tables and procedures etc. This back up can server as an independent entity that can be restored in different systems with just the base SQL server installed.

Transaction log backup: This is the mechanism of backing up the transaction logs that have been maintained in the server. This way the details of the database getting updated is obtained. This cannot be a stand-alone back up mechanism. But can save a lot of time if we already have the file system related to the DB backed up on the new deployment server.

Differential backup: This is a subset of the complete backup, where only the modified datasets are backed up. This can save the time when we are just trying to maintain a backup server to main server.

File backup: This is the quickest way to take the backup of entire database. Instead of taking in the data actually stored in DB, the files are backed up and the file system thus obtained when combined with the transaction logs of the original system will render the database that we are trying to back up.

.Q What are the different levels of isolation?

Answer: The isolation represents the way of separating the database from the effects of network accesses, thereby maintaining the consistency. The different levels of isolation are:

read committed: This level of isolation uses the shared locks and the reads to the database give the constant and consistent values.

read uncommitted: No locks implemented. This is the least effective isolation level.

repeatable read: There are lock over the rows and values but the updates are maintained as a separate phantom row which is the next set of values for the specific record. Values can change within a specific transaction of a SQL function.

SERIALIZABLE reads: This is the implementation of pure lock mechanism where one specific transaction is not allowed access to specific record before another one completes.

Q.What are the steps to take to improve performance of a poor performing query?Answer: Maximum use of indexes, stored procures should be done. Avoid excessive use of complicated joins and cursors. Avoid using conditional operators using columns of different tables. Make use of computed columns and rewriting the query. know more at SQL SERVER DBA online trainingQ.What is normalization? Explain different levels of normalization?Answer: 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.Q.What is denormalization and when would you go for it?Answer: 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.Q.How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?Answer: 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.Q.What's the difference between a primary key and a unique key?Answer: 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.Q.What are user defined datatypes and when you should go for them?Answer: 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. know more at SQL SERVER DBA trainingQ.What is bit datatype and what's the information that can be stored inside a bit column?Answer: 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.Q.Define candidate key, alternate key, composite key.Answer: 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.Q.What are defaults? Is there a column to which a default can't be bound?Answer: 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. Q.What is a transaction and what are ACID properties? Answer: 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. know more at SQL Server dba online training from IndiaQ.Explain different isolation levelsAnswer: 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.Q.What type of Index will get created after executing the above statement?Answer: Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.Q.What's the maximum size of a row?Answer: 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".Q.What is lock escalation?Answer: 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.Q.What's the difference between DELETE TABLE and TRUNCATE TABLE commands?Answer: 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.Q.Explain the storage models of OLAPAnswer: 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?Q.What are constraints? Explain different types of constraints.Answer: 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 KEYQ.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?Answer: 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. Database administrationQ.What is RAID and what are different types of RAID configurations?Answer: 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 homepageQ.What are the steps you will take to improve performance of a poor performing query?Answer: 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.

Q.What are the steps you will take, if you are tasked with securing an SQL Server?Answer: 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.Q.What is a deadlock and what is a live lock? How will you go about resolving deadlocks?Answer: 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.Q.What is blocking and how would you troubleshoot it?Answer: 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.Q.Explain CREATE DATABASE syntaxAnswer: 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.Q.How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?Answer: 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.Q.As a part of your job, what are the DBCC commands that you commonly use for database maintenance?Answer: DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc .Q.What are statistics, under what circumstances they go out of date, how do you update them?Answer: 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 versionQ.What are the different ways of moving data/databases between servers and databases in SQL Server?Answer: 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. know more at SQL SERVER DBA online course Q.What is database replicaion? What are the different types of replication you can set up in SQL Server?Answer:  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. Database programming (top)Q.What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?Answer:  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