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13.2.11.6 Subqueries with EXISTS or NOT EXISTS
If a subquery returns any rows at all, EXISTS
is
subqueryTRUE, and NOT EXISTS
is
subqueryFALSE. For example:
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?
What kind of store is present in no cities?
What kind of store is present in all cities?
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?”
Document created the 26/06/2006, last modified the 26/10/2018
Source of the printed document:https://www.gaudry.be/en/mysql-rf-exists-and-not-exists-subqueries.html
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