SQL---Subqueries

Sub-query, also called nested query.

A query statement is used as a result set for other SQL statements, just as a query statement that is treated as a result set is called a subquery, as with a normal table.

There are two types of sub-queries:

One is to return only a single-valued subquery, at which point it can be used where a single value can be used, when a subquery can be considered a function that has a return value;

Another is a subquery that returns a column of values, at which point the subquery can be thought of as a data table that is temporarily present in memory.

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SQL---Subqueries