有时用别的方式而不是子查询可以获得更高的性能 :
For example:
SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);
改写:
SELECT DISTINCT t1.* FROM t1, t2 WHERE t1.id=t2.id;
查询:
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);
改写:
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id WHERE table2.id IS NULL;
LEFT [OUTER] JOIN.
原文：http://www.cnblogs.com/onlysun/p/4512270.html