SQL语句的执行顺序

一.sql语句的执行顺序

 (8)SELECT  (9) DISTINCT  (11)  <TOP_specification> <select_list> 

 (1)  FROM <left_table>  

 (3) <join_type> JOIN <right_table> 

 (2) ON <join_condition> 

 (4) WHERE <where_condition> 

 (5) GROUP BY <group_by_list> 

 (6) WITH {CUBE | ROLLUP} 

 (7) HAVING <having_condition> 

 (10) ORDER BY <order_by_list>

二.测试left join和 where的执行顺序

 创建测试表
 create table t1(id int,feild int);
 create table t2(id int,feild int);

 insert into t1 values(1,1);
 insert into t1 values(1,2);
 insert into t1 values(1,3);
 insert into t1 values(1,4);
 insert into t1 values(2,1);
 insert into t1 values(2,2);

 insert into t2 values(1,1);
 insert into t2 values(1,2);
 insert into t2 values(1,5);
 insert into t2 values(1,6);
 insert into t2 values(2,1);
 insert into t2 values(2,3);

 测试 语句：
 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id
 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id and t1.id=1
 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id where t1.id=1
 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id and t2.id=1
 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id where t2.id=1
 select t1.*,t2.* from t1 inner join t2 on t1.id=t2.id and t2.id=1

结论：

结论：取t1表的第一行,按谓词‘on’中的条件扫瞄t2表,如果满足条件,就加入返回结果表，不满足条件则只返回t1.
然后取t1表的第二行,按谓词‘on’中的条件扫瞄t2表,如果满足条件,就加入返回结果表，不满足条件则只返回t1.
重复以上过程,直到t1表扫描结束

结论：

结论：取t1表的第一行,按谓词‘on’中的条件扫瞄t1,t2表,如果满足条件,就加入返回结果表，不满足条件则只返回t1.
然后取t1表的第二行,按谓词‘on’中的条件扫瞄t2表,如果满足条件,就加入返回结果表，不满足条件则只返回t1.
重复以上过程,直到t1表扫描结束

结论：

结论：

结论：

结论：