A subquery is a SELECT statement that is nested inside another SELECT, INSERT, UPDATE, or DELETE statement, or another subquery.
If a subquery matches no rows, it evaluates to NULL.
The SELECT statement must be enclosed in parentheses, and must contain one and only one SELECT list item. When used as an expression, a subquery is generally allowed to return only one value.
A subquery can be used anywhere that a column name can be used. For example, a subquery can be used in the SELECT list of another SELECT statement.
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