Oracle 5.0 Reference Manual page 1122

an empty result set. The expression is unknown (that is, NULL) if the subquery produces no rows. An
error occurs if the subquery produces multiple rows because a row subquery can return at most one
row.
The expressions
equivalent. The row constructor and the row returned by the subquery must contain the same number
of values.
A row constructor is used for comparisons with subqueries that return two or more columns. When
a subquery returns a single column, this is regarded as a scalar value and not as a row, so a row
constructor cannot be used with a subquery that does not return at least two columns. Thus, the
following query fails with a syntax error:
SELECT * FROM t1 WHERE ROW(1) = (SELECT column1 FROM t2)
Row constructors are legal in other contexts. For example, the following two statements are
semantically equivalent:
SELECT * FROM t1 WHERE (column1,column2) = (1,1);
SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;
Prior to MySQL 5.0.26, only the second of the preceding two expressions could be optimized. (Bug
#16081)
The following query answers the request, "find all rows in table
SELECT column1,column2,column3
FROM t1
WHERE (column1,column2,column3) IN
(SELECT column1,column2,column3 FROM t2);
13.2.9.6. Subqueries with
If a subquery returns any rows at all,
FALSE. For example:
SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
Traditionally, an
or anything at all. MySQL ignores the
column1
difference.
For the preceding example, if
condition is TRUE. This is actually an unlikely example because a
EXISTS
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
• What kind of store is present in no cities?
SELECT DISTINCT store_type FROM stores
WHERE NOT EXISTS (SELECT * FROM cities_stores
• 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
Subquery Syntax
and
(1,2) ROW(1,2)
or
EXISTS NOT EXISTS
EXISTS subquery
subquery starts with
EXISTS
contains any rows, even rows with nothing but
t2
WHERE cities_stores.store_type = stores.store_type);
WHERE cities_stores.store_type = stores.store_type);
are sometimes called row constructors. The two are
is TRUE, and
*, but it could begin with
SELECT
list in such a subquery, so it makes no
SELECT
1102
that also exist in table t2":
t1
NOT EXISTS subquery
SELECT 5
NULL
[NOT] EXISTS
is
or
SELECT
values, the
subquery