将子查询重写为联合

Sometimes there are other ways to test membership in a set of values than by using a subquery. Also, on some occasions, it is not only possible to rewrite a query without a subquery, but it can be more efficient to make use of some of these techniques rather
than to use subqueries. One of these is the

IN()

construct:

For example, this query:

SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);

Can be rewritten as:

SELECT DISTINCT t1.* FROM t1, t2 WHERE t1.id=t2.id;

The queries:

SELECT * FROM t1 WHERE id NOT IN (SELECT id FROM t2);
SELECT * FROM t1 WHERE NOT EXISTS (SELECT id FROM t2 WHERE t1.id=t2.id);

Can be rewritten as:

SELECT table1.*
  FROM table1 LEFT JOIN table2 ON table1.id=table2.id
  WHERE table2.id IS NULL;

A

LEFT [OUTER] JOIN

can be faster than
an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things. Today, MySQL Server and many
other modern database systems offer a wide range of outer join types.

MySQL Server supports multiple-table

DELETE

statements
that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. Multiple-table

UPDATE

statements
are also supported. See Section 13.2.2,
“

DELETE

Syntax”, and Section 13.2.11,
“

UPDATE

Syntax”.