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MySQL 5.6 Reference Manual  /  ...  /  The Subquery as Scalar Operand

13.2.10.1 The Subquery as Scalar Operand

In its simplest form, a subquery is a scalar subquery that returns a single value. A scalar subquery is a simple operand, and you can use it almost anywhere a single column value or literal is legal, and you can expect it to have those characteristics that all operands have: a data type, a length, an indication that it can be NULL, and so on. For example:

CREATE TABLE t1 (s1 INT, s2 CHAR(5) NOT NULL);
INSERT INTO t1 VALUES(100, 'abcde');
SELECT (SELECT s2 FROM t1);

The subquery in this SELECT returns a single value ('abcde') that has a data type of CHAR, a length of 5, a character set and collation equal to the defaults in effect at CREATE TABLE time, and an indication that the value in the column can be NULL. Nullability of the value selected by a scalar subquery is not copied because if the subquery result is empty, the result is NULL. For the subquery just shown, if t1 were empty, the result would be NULL even though s2 is NOT NULL.

There are a few contexts in which a scalar subquery cannot be used. If a statement permits only a literal value, you cannot use a subquery. For example, LIMIT requires literal integer arguments, and LOAD DATA INFILE requires a literal string file name. You cannot use subqueries to supply these values.

When you see examples in the following sections that contain the rather spartan construct (SELECT column1 FROM t1), imagine that your own code contains much more diverse and complex constructions.

Suppose that we make two tables:

CREATE TABLE t1 (s1 INT);
INSERT INTO t1 VALUES (1);
CREATE TABLE t2 (s1 INT);
INSERT INTO t2 VALUES (2);

Then perform a SELECT:

SELECT (SELECT s1 FROM t2) FROM t1;

The result is 2 because there is a row in t2 containing a column s1 that has a value of 2.

A scalar subquery can be part of an expression, but remember the parentheses, even if the subquery is an operand that provides an argument for a function. For example:

SELECT UPPER((SELECT s1 FROM t1)) FROM t2;

User Comments
  Posted by mrfox on February 6, 2007
when the same subquery is used several times, mysql does not use this fact to optimize the query, so be careful not to run into performance problems.

example:

SELECT
col0,
(SELECT col1 FROM table1 WHERE table1.id = table0.id),
(SELECT col2 FROM table1 WHERE table1.id = table0.id)
FROM
table0
WHERE ...

the join of table0 with table1 is executed once for EACH subquery, leading to very bad performance for this kind of query.
  Posted by Rami Jamleh on June 3, 2012
Oracle says that you can left join via scalar sub-queries
e.g
create table x (id int auto_increment primary key,name varchar(20),yid int);

create table y (id int auto_increment primary key,name varchar(20));

insert into x(name,yid) values('Alex',null),('Marvin',1),('Bob',null);
insert into y(name) values('GO'),('GOO'),('GOOO');

watch this now
select id,name,(select name from y where x.yid = y.id) from x;
which is equivalent to
select x.id,x.name,y.name
from x left join y on (x.yid = y.id);

and they say outer joins may have negative impact on performance
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