13.2.8.6. 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?”
