A subquery is a SELECT statement that returns values used to satisfy the comparison operation. The subquery must be enclosed in parentheses, and does not end with a semicolon.
The subquery must refer to at least one table. A table that is in the WHERE clause, but that is not referred to in any other parts of the subquery, is not applicable.
A comparison operation may be used with a subquery whether or not a quantifier is used. If a quantifier is not used, however, then an error condition results if the subquery returns more than one value.
If a subquery returns no values, and if a quantifier is not used, the result of the comparison is false. Therefore, if the following form is used, the subquery must return no values (in which case the comparison evaluates to false), or one value.
expression > (subquery)
With the following form, subquery must select the same number of expressions as are specified in the expression list.
The two expression lists are equal if each of the respective expressions are equal.
If the respective expressions are not equal, then the result of the comparison is determined by comparing the first pair of expressions (from the left) for which the comparison is not true.
A subquery in a comparison operation cannot specify a SELECT AND CONSUME statement.