4.2.4. Quantified Subquery Predicates

A quantifier is a logical operator that sets the number of objects for which this assertion is true. It is not a numeric quantity, but a logical one that connects the assertion 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:

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

ALL

Available

DSQL, PSQL, ESQL

Syntax

  1. <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.

  1. SELECT c1.*
  2. FROM Customers c1
  3. WHERE c1.rating > ALL
  4.       (SELECT c2.rating
  5.        FROM Customers c2
  6.        WHERE c2.city = 'Paris')

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 0 times, i.e., for every row in the set.

ANY and SOME

Available

DSQL, PSQL, ESQL

Syntax

  1. <value> <op> {ANY | SOME} (<select_stmt>)

The quantifiers ANY and SOME are identical in their behaviour. Apparently, both are present in the SQL standard so that they could be used interchangeably in order to improve the readability of operators. When the ANY or the SOME quantifier is used, the predicate is TRUE if any of the values returned by the subquery satisfies the condition in the predicate of the main query. If the subquery would return no rows at all, the predicate is automatically considered as FALSE.

Example

Show only those clients whose ratings are higher than those of one or more clients in Rome.

  1. SELECT *
  2. FROM Customers
  3. WHERE rating > ANY
  4.       (SELECT rating
  5.        FROM Customers
  6.        WHERE city = 'Rome')