Question

您好，请您告诉我哪一个更优化：

select a.*,b.* from a join b on a.id=b.id and a.name='test';

select a.*,b.* from a join b on a.id=b.id where a.name='test';

由于

Answer 1

在我看来，语法良好，如果不是那些＆＃39;并且之后有一系列WHERE子句，则更容易阅读查询。

由于您使用内部联接，因此两者之间没有区别。内部联接仅显示联接中存在完全匹配的位置。

如果你要使用左连接，事情会有所不同。

Answer 2

在最简单的情况下，两者都是等价的。为了证明，你可以看看两个执行计划

explain select a.*,b.* from a join b on a.id=b.id and a.name='test'

给出

+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+  
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |  
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+  
|  1 | SIMPLE      | a     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where                    |  
|  1 | SIMPLE      | b     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where; Using join buffer |  
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+  
2 rows in set (0.00 sec)

然后是

explain select a.*,b.* from a join b on a.id=b.id where a.name='test'

也给出了

+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | a     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where                    |
|  1 | SIMPLE      | b     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)

另见SQLFiddle

加入vs where-SQL

2 个答案: