# Introduction To Subquery
__*Subquery definition*__
+ A subquery is a query within a query
+ A powerful tool that can be used in many SQL statements
+ Alternatives to JOIN
+ Also known as an inner query or a nested query
+ Execute as part of a main query or an external query
+ Always in parentheses: `( )`   
+ Allowed only in certain parts of a statement, depending on the data it return

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