Subqueries in SELECT Statements

You can construct a SELECT statement with a subquery to replace two separate SELECT statements.

Subqueries in SELECT statements allow you to perform the following actions:

• Compare an expression to the result of another SELECT statement
• Determine whether the results of another SELECT statement include an expression
• Determine whether another SELECT statement selects any rows

An optional WHERE clause in a subquery is often used to narrow the search condition.

A subquery selects and returns values to the first or outer SELECT statement. A subquery can return no value, a single value, or a set of values, as follows:

• If a subquery returns no value, the query does not return any rows. Such a subquery is equivalent to a NULL value.
• If a subquery returns one value, the value is in the form of either one aggregate expression or exactly one row and one column. Such a subquery is equivalent to a single number or character value.
• If a subquery returns a list or set of values, the values represent either one row or one column.