You can place a ScalarSubquery anywhere an Expression is
permitted. A ScalarSubquery turns a SelectExpression result
into a scalar value because it returns only a single row and column value.
The
query must evaluate to a single row with a single column.
Sometimes also called an expression subquery.
Examples
-- avg always returns a single value, so the subquery is
-- a ScalarSubquery
SELECT NAME, COMM
FROM STAFF
WHERE EXISTS
(SELECT AVG(BONUS + 800)
FROM EMPLOYEE
WHERE COMM < 5000
AND EMPLOYEE.LASTNAME = UPPER(STAFF.NAME)
)
-- Introduce a way of "generating" new data values,
-- using a query which selects from a VALUES clause (which is an
-- alternate form of a fullselect).
-- This query shows how a table can be derived called "X" having
-- 2 columns "R1" and "R2" and 1 row of data.
SELECT R1,R2
FROM (VALUES('GROUP 1','GROUP 2')) AS X(R1,R2)
