Quantified Subquery Predicates

Data Types and SubtypesData Definition (DDL) Statements

Quantified Subquery Predicates

A quantifier is a logical operator that sets the number of objects for which this condition is true.It is not a numeric quantity, but a logical one that connects the condition with the full set of possible objects.Such predicates are based on logical universal and existential quantifiers that are recognised in formal logic.

In subquery expressions, quantified predicates make it possible to compare separate values with the results of subqueries;they have the following common form:

<value expression> <comparison operator> <quantifier> <subquery>

`ALL`

Syntax

<value> <op> ALL (<select_stmt>)

When the ALL quantifier is used, the predicate is TRUE if every value returned by the subquery satisfies the condition in the predicate of the main query.

Example

Show only those clients whose ratings are higher than the rating of every client in Paris.

SELECT c1.*
FROM Customers c1
WHERE c1.rating > ALL
      (SELECT c2.rating
       FROM Customers c2
       WHERE c2.city = 'Paris')

Important

If the subquery returns an empty set, the predicate is TRUE for every left-side value, regardless of the operator.This may appear to be contradictory, because every left-side value will thus be considered both smaller and greater than, both equal to and unequal to, every element of the right-side stream.

Nevertheless, it aligns perfectly with formal logic: if the set is empty, the predicate is true for every row in the set.