MySql优化子查询

SELECT * FROM t1 WHERE t1.column1 IN
  (SELECT column1 FROM t2 ORDER BY column1);
SELECT * FROM t1 WHERE t1.column1 IN
  (SELECT DISTINCT column1 FROM t2);
SELECT * FROM t1 WHERE EXISTS
  (SELECT * FROM t2 LIMIT 1);//limit关键字不在含有in关键字的子查询中（用exists代替）

SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN (
  SELECT column1 FROM t2);

代替:

SELECT DISTINCT t1.column1 FROM t1, t2
  WHERE t1.column1 = t2.column1;

SELECT * FROM t1
  WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1 FROM t2);

代替:

SELECT * FROM t1
  WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1 FROM t2);

For another example:

SELECT (SELECT column1 + 5 FROM t1) FROM t2;

代替:

SELECT (SELECT column1 FROM t1) + 5 FROM t2;

SELECT * FROM t1
  WHERE (column1,column2) IN (SELECT column1,column2 FROM t2);

代替:

SELECT * FROM t1
  WHERE EXISTS (SELECT * FROM t2 WHERE t2.column1=t1.column1
                AND t2.column2=t1.column2);

SELECT * FROM t1
  WHERE t1.col_name = (SELECT a FROM t2 WHERE b = some_const);

代替:

SELECT * FROM t1
  WHERE t1.col_name IN (SELECT a FROM t2 WHERE b = some_const);

... IN (SELECT indexed_column FROM single_table ...)

value {ALL|ANY|SOME} {> | < | >= | <=} (uncorrelated subquery)

For example,:

WHERE 5 > ALL (SELECT x FROM t)