对多表联合查询的SQl语句的改进，大幅提高查询性能

常用的多表联合查询的sql语句：

select table1.ID from table1 inner join table2 on table1.ID=table2.ID
 where  table2.col2='xxx'

 

改进后：

select a.ID from table1 a,
(select col1,ID from table2 where col2='xxx') b
 where a.ID=b.ID