MySql优化子查询

用子查询语句来影响子查询中产生结果rows的数量和顺序. For example:

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）

代替和子查询做join操作. For example:

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;

一些子查询会被改写成join连接为了兼容不支持子查询的老版本.然而，在一些情况下改写子查询为join操作会提高性能 ;

去掉在子查询中出现的外部语句. For example:

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;

用行子查询代替一个相关子查询. For example:

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);

用 NOT (a = ANY (...)) 代替 a <> ALL (...).
用 x = ANY (table containing (1,2))代替 x=1 OR x=2.
用 = ANY 代替 EXISTS.

因为不相关的子查询通常返回一行结果, IN 通常慢于 =. For example:

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);

MySQL 执行不相关的子查询一次. 用 explain确保一个子查询是真正的不相关的.
MySQL改写 IN, ALL, ANY, and SOME 子查询尝试提高select的列在子查询中加索引的可能性 .
MySQL 代替用带Index查找功能，explain语句描述为一种特别的join(unqie subquery 或者index subquery)子查询（如下面形式）:
```
... IN (SELECT indexed_column FROM single_table ...)
```
MySQL 增前了表达式（以下形式调用（min() or max()）, 除非null值或者空集合:
```
value {ALL|ANY|SOME} {> | < | >= | <=} (uncorrelated subquery)
```
For example,:
```
WHERE 5 > ALL (SELECT x FROM t)
```
可能被优化成：WHERE 5 > (SELECT MAX(x) FROM t)