If a subquery returns any rows at all, EXISTS
is
subqueryTRUE, and NOT EXISTS
is
subqueryFALSE. 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?”