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. Otherwise, the result column is unnamed.
See fullselect for more information.
A scalar fullselect is not allowed if the query specifies:
If the scalar fullselect is a subselect, it is also referred to as a scalar subselect. See subselect for more information.