Subquery Limitations

There are a number of limitations regarding subqueries, which are discussed below.

The following tables and data will be used in the examples that follow:

CREATE TABLE staff(name VARCHAR(10),age TINYINT);

CREATE TABLE customer(name VARCHAR(10),age TINYINT);

INSERT INTO staff VALUES 
('Bilhah',37), ('Valerius',61), ('Maia',25);

INSERT INTO customer VALUES 
('Thanasis',48), ('Valerius',61), ('Brion',51);

ORDER BY and LIMIT

To use ORDER BY or limit LIMIT in subqueries both must be used.. For example:

SELECT * FROM staff WHERE name IN (SELECT name FROM customer ORDER BY name);
+----------+------+
| name     | age  |
+----------+------+
| Valerius |   61 |
+----------+------+

is valid, but

SELECT * FROM staff WHERE name IN (SELECT NAME FROM customer ORDER BY name LIMIT 1);
ERROR 1235 (42000): This version of MariaDB doesn't 
  yet support 'LIMIT & IN/ALL/ANY/SOME subquery'

is not.

Modifying and Selecting from the Same Table

It's not possible to both modify and select from the same table in a subquery. For example:

DELETE FROM staff WHERE name = (SELECT name FROM staff WHERE age=61);
ERROR 1093 (HY000): Table 'staff' is specified twice, both 
  as a target for 'DELETE' and as a separate source for data

Row Comparison Operations

There is only partial support for row comparison operations. The expression in

expr op {ALL|ANY|SOME} subquery,

must be scalar and the subquery can only return a single column.

However, because of the way IN is implemented (it is rewritten as a sequence of = comparisons and AND), the expression in

expression [NOT] IN subquery

is permitted to be an n-tuple and the subquery can return rows of n-tuples.

For example:

SELECT * FROM staff WHERE (name,age) NOT IN (
  SELECT name,age FROM customer WHERE age >=51]
);
+--------+------+
| name   | age  |
+--------+------+
| Bilhah |   37 |
| Maia   |   25 |
+--------+------+

is permitted, but

SELECT * FROM staff WHERE (name,age) = ALL (
  SELECT name,age FROM customer WHERE age >=51
);
ERROR 1241 (21000): Operand should contain 1 column(s)

is not.

Correlated Subqueries

Subqueries in the FROM clause cannot be correlated subqueries. They cannot be evaluated for each row of the outer query since they are evaluated to produce a result set during when the query is executed.

Stored Functions

A subquery can refer to a stored function which modifies data. This is an extension to the SQL standard, but can result in indeterminate outcomes. For example, take:

SELECT ... WHERE x IN (SELECT f() ...);

where f() inserts rows. The function f() could be executed a different number of times depending on how the optimizer chooses to handle the query.

This sort of construct is therefore not safe to use in replication that is not row-based, as there could be different results on the master and the slave.

_{This page is licensed: CC BY-SA / Gnu FDL}