Example of NOT EXISTS Operator - SQL

Example is a translation into SQL of the corresponding example, which is included there merely to show that for any scalar comparison there is an alternative formulation using IS_EMPTY.

Example: MAX_ENROLMENTS expressed using an invocation of NOT EXISTS

CREATE ASSERTION MAX_ENROLMENTS_alternative1

CHECK (NOT EXISTS (SELECT *

FROM (VALUES (SELECT COUNT (*)

FROM IS_ENROLLED_ON)) AS

V (N)

WHERE V.N > 20000));

Explanation

• VALUES ( SELECT COUNT(*) FROM IS_ENROLLED_ON ) denotes a table with just one column, unnamed, in whose single row the value of that column is the number of rows in the current value of IS_ENROLLED_ON. The SELECT expression is parenthesized to make it into a scalar subquery and given as the argument to an invocation of VALUES, which makes the number denoted by that scalar subquery into a one-row, one-column table. (Actually, it might be safer to place an extra pair of parentheses around the SELECT expression here. Although VALUES 1 and VALUES (1) are equivalent, it might not be clear as to which role the single parentheses are taking: do they denote a scalar subquery, as I have assumed, or are they the optional ones surrounding a single table expression? If the latter, we would expect a syntax error.)
• AS V (N) defines the range variable V to refer to what in this case is just the single row of that table, and also assigns the name N to its only column.
•  WHERE V.N > 20000 operates on that one-row, one-column table to yield a table of heading (N INTEGER) that is empty if and only if the single row in that one-row, one-column table fails to satisfy the condition N > 20000. Thus, the result is empty only when the number of enrolments is in fact no greater than the maximum allowed.