Oct 11, 2016

Interview Questions and answers - SQL Server Day 7

Question: What are ACID properties in SQL Server?

Answer: The ACID rules of transaction in any database ensure the reliability of data in all transactions in the database.

Atomicity
Atomicity shows the ability of database to assure that either all tasks of a transaction are performed or none of them.

Consistency
Consistency assure that database remain consistent state before the transaction and after the transaction is over.

Isolation
Isolation assures that other operation can't be performed in an intermediate state during a transaction. As a result, transactions that run concurrently appear to be serialized.

Durability
Durability is responsible to notify the transaction is succeed; the transaction will persist, not undone. It will survive system failure and the database system has checked the integrity constraints and no need to abort the transaction.
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Question: What is TRANSACTION in SQL Server?

Answer: Transaction is the collection of T-SQL statements which executes in single unit. Transaction begins with a specific T-SQL statement and ends when all T-SQL statements execute successfully. If any statements fail then transaction fails. Transaction has only two types of results success or failed.
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Question: What are COMMIT and ROLLBACK in SQL Server?

Answer: COMMIT statement use to committed transaction to save changes in database successfully.
ROLLBACK statement use to stop current transaction and undone the changes made by transaction.
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Question: What is a WITH(NOLOCK) in SQL Server?

Answer: WITH(NOLOCK) used in SELECT statement to give the priority to select statement. WITH(NOLOCK) unlock the table for SELECT statement to stop another process.
Example
SELECT * FROM TBL WITH(NOLOCK)

Question: GO command in SQL server?

Answer: GO command indicates the end the SQL statements it is used when there are multiple statements to be executed but sent as a batch.

Question: What is difference between UNION and UNION ALL in SQL Server?

Answer: UNION operator is use to combine multiple result set into one result set but remove any duplicate rows. Basically UNION use to performing a DISTINCT operation across all columns in the result set. UNION operator has extra overhead of removing duplicate rows and sorting result.


UNION ALL operator use to combine multiple results set into one result set but it does not remove any duplicate result. Actually this is not remove duplicate rows so it is faster than UNION operator, if you want to combine multiple results and without duplicate records then use UNION. For more details click here