一.sql语句的执行顺序
1 (8)SELECT (9) DISTINCT (11) <TOP_specification> <select_list> 2 3 (1) FROM <left_table> 4 5 (3) <join_type> JOIN <right_table> 6 7 (2) ON <join_condition> 8 9 (4) WHERE <where_condition> 10 11 (5) GROUP BY <group_by_list> 12 13 (6) WITH {CUBE | ROLLUP} 14 15 (7) HAVING <having_condition> 16 17 (10) ORDER BY <order_by_list>
二.测试left join和 where的执行顺序
1 创建测试表 2 create table t1(id int,feild int); 3 create table t2(id int,feild int); 4 5 insert into t1 values(1,1); 6 insert into t1 values(1,2); 7 insert into t1 values(1,3); 8 insert into t1 values(1,4); 9 insert into t1 values(2,1); 10 insert into t1 values(2,2); 11 12 insert into t2 values(1,1); 13 insert into t2 values(1,2); 14 insert into t2 values(1,5); 15 insert into t2 values(1,6); 16 insert into t2 values(2,1); 17 insert into t2 values(2,3);
1 测试 语句： 2 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id 3 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id and t1.id=1 4 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id where t1.id=1 5 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id and t2.id=1 6 select t1.*,t2.* from t1 left join t2 on t1.id=t2.id where t2.id=1 7 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：