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Interview Questions and answers - SQL Server Day 6

Question: When we used UPDATE_STATISTICS commands in SQL server?

Answer: When bulk changes done in your data base like INSERT, UPDATE and DELETE because bulk changes may increase your index view fragmentation. UPDATE_STATISTICS Updates query optimization statistics on a table or indexed view.

Question: What is view in SQL Server?

Answer: View is a virtual table which contains one or more than one fields of more than one physical table. View is used for de-normalization purpose. This is used to join multiple tables and get the data.

Question: What is the property of relational table in SQL Server?
Answer:
  1. Values are atomic.
  2. Column values are of the same kind.
  3. Each row is unique.
  4. The sequence of columns is insignificant.
  5. The sequence of rows is insignificant.
  6. Each column must have a unique name.


Question: Explain different type of lock in SQL Server?

Answer: Following locks are available in SQL Server.

Shared (S)
Used for read operations those do not change or update data, such as a SELECT statement.

Update (U)
Used on resources that can be updated. Prevents a common form of deadlock that occurs when multiple sessions are reading, locking and potentially updating resources later.

Exclusive (X)
Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time.

Intent
Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).

Schema
Used when an operation dependent on the schema of a table is executing. The types of schema locks are: schema modification (Sch-M) and schema stability (Sch-S).
Bulk Update (BU)
Used when bulk copying data into a table and the TABLOCK hint is specified.

Key-range
Protects the range of rows read by a query when using the serializable transaction isolation level. Ensures that other transactions cannot insert rows that would qualify for the queries of the serializable transaction if the queries were run again.

Question: What is DBCC command in SQL Server?

Answer: DBCC stands for Database Consistency Checker.These commands are used to check the consistency of the database like validation tasks, maintenance and status checks.

For example –

DBCC CHECKALLOC – Responsible to check that all pages are correctly allocated in database.

DBCC CHECKDB – Responsible to check the integrity & allocation of specific objects in a database.

DBCC SQLPERF- Responsible to show transaction log statistics.


DBCC SQLFILEGROUP – Responsible to check all the tables file group for any design.

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What is difference between UNION and UNION ALL in SQL Server

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UNION ALL operator use to combine multiple results set into one result set but it does not remove any duplicate result. Actually, this does not remove duplicate rows so it is faster than the UNION operator. If you want to combine multiple results and without duplicate records then use UNION otherwise UNION ALL is better.
Following some rules for using UNION/UNION ALL operator
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Syntax:

{ $add: [ <expression1>, <expression2>, ... ] }

Note:  If one of the argument is date $add operator treats to other arguments as milliseconds to add to the date.
Example: Suppose we have a Test collection as below.

{"Title" : "Add day to ISODate in MongoBD","CreatedDate" : ISODate("2016-07-07T08:00:00.000Z")}

Query to add 2 days in CreatedDate

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Result:

{ "_id" : ObjectId("579a1567ac1b3f3732483de0"), "Title" : "Add day to ISODate in MongoBD", "AddedDate" : ISODate("2016-07-09T08:00:00.000Z") }

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Following query may help you to resolve your issue.

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Query to find foreign key relationship and name of the constraints for each table in the database

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Result set
name            Schema ------------------------------------------------ Codefari       dbo
(1 row(s) affected)
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CREATESCHEMA Blog GO ALTERSCHEMA Blog TRANSFER dbo.Codefari GO
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Like in MongoDB Query

In SQL, 'LIKE' query look like as below
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db.USERS.find({"FNAME":/DILIP/})
Same as
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$regex in MongoDB