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SQL Subqueries

  • SQL Subqueries are the nested queries, there will be one main query inside that, we might have one or more subqueries.

  • subqueries are called as inner queries.

  • mainqueries are called as outer queries.

Subqueries - IN Clause

Syntax 

SELECT 
       *
FROM
      table_1   t1
WHERE
           column_name in (inner_query or subquery)

Subqueries - EXISTS Clause

Syntax 

SELECT 
       *
FROM
      table_1   t1
WHERE
           EXISTS (inner_query or subquery)

IN Clause

  • It search among the list.

  • It is faster for smaller data.

EXISTS Clause

  • It checks for the existence.

  • It is faster for larger amount of data.

  • All the above queries which we can write using subqueries can be written using joins as well.

  • Sometimes it is efficient to use join and sometimes it is good to use subqueries. with time we will understand when to use joins and when to use subqueries.

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