MySQL改写子查询成Join

   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.