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13.2.11.12 Rewriting Subqueries as Joins

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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:

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

Can be rewritten as:

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

The queries:

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

Can be rewritten as:

  1. SELECT table1.*
  2.   FROM table1 LEFT JOIN table2 ON table1.id=table2.id
  3.   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 13.2.12, “UPDATE Syntax”.


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Document created the 26/06/2006, last modified the 26/10/2018
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