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12.2.8.7 Correlated Subqueries »
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12.2.8.6. EXISTS and NOT EXISTS

If a subquery returns any rows at all, EXISTS subquery is TRUE, and NOT EXISTS subquery is FALSE. For example: 

SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);

Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.

For the preceding example, if t2 contains any rows, even rows with nothing but NULL values, the EXISTS condition is TRUE. This is actually an unlikely example because a [NOT] EXISTS subquery almost always contains correlations. Here are some more realistic examples:

  • What kind of store is present in one or more cities? 

    SELECT DISTINCT store_type FROM stores
  WHERE EXISTS (SELECT * FROM cities_stores
                WHERE cities_stores.store_type = stores.store_type);

  • What kind of store is present in no cities? 

    SELECT DISTINCT store_type FROM stores
  WHERE NOT EXISTS (SELECT * FROM cities_stores
                    WHERE cities_stores.store_type = stores.store_type);

  • What kind of store is present in all cities? 

    SELECT DISTINCT store_type FROM stores s1
  WHERE NOT EXISTS (
    SELECT * FROM cities WHERE NOT EXISTS (
      SELECT * FROM cities_stores
       WHERE cities_stores.city = cities.city
       AND cities_stores.store_type = stores.store_type));

The last example is a double-nested NOT EXISTS query. That is, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers the question “does a city exist with a store that is not in Stores”? But it is easier to say that a nested NOT EXISTS answers the question “is x TRUE for all y?”


User Comments

Posted by ersin yilmaz on February 24 2004 3:34pm[Delete] [Edit]

Last example (using double negation) can be interpreted as follows:

Store 'si' is in all cities if and only if for this store we can not find a city cj such that 'si' does not exist.

so the second select statement SELECT * FROM cities WHERE NOT EXISTS is applied to detemine that.

best

Posted by Radu Borza on February 9 2005 12:21am[Delete] [Edit]

EXISTS function provides a simple way to find intersection between tables (INTERSECT operator from relational model).

If we have table1 and table2, both having id and value columns, the intersection could be calculated like this:

SELECT * FROM table1 WHERE EXISTS(SELECT * FROM table2 WHERE table1.id=table2.id AND table1.value=table2.value)

Posted by Aaron Gussman on June 16 2006 1:18pm[Delete] [Edit]

EXISTS is supported from 4.1 on. Earlier versions of MySQL can try rewriting the query using a LEFT JOIN.

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