Scalar fullselect

A scalar fullselect as supported in an expression is a fullselect, enclosed in parentheses, that returns a single row consisting of a single column value. If the fullselect does not return a row, the result of the expression is the null value. If the select list element is an expression that is simply a column name, the result column name is based on the name of the column. See fullselect for more information.

A scalar fullselect is not allowed if the query specifies:

A scalar subselect as supported in an expression is a subselect, enclosed in parentheses, that returns a single row consisting of a single column value. If the subselect does not return a row, the result of the expression is the null value. If the select list element is an expression that is simply a column name, the result column name is based on the name of the column. See fullselect for more information.