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SQL Interview Q & A

A Series of SQL Server basics to advanced level SQL Interview questions and answers that are commonly asked in interivew.

What is Correlated Subquery in SQL?

A correlated subquery is a subquery that uses the values of the outer query  and it is evaluated for each row processed by the outer query 

A correlated subquery  always depends on outer query for its value. Because of this dependency, a correlated subquery cannot be executed independently as a simple subquery .

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Primary Key VS Composite Key ?

A PRIMARY KEY constraint uniquely identifies each record in a table .

A COMPOSITE KEY is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness is guaranteed, but when it taken individually it does not guarantee uniqueness.

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