MYSQL 表左连接 ON AND 和ON WHERE 的区别

Posted 2020-10-26 梦想远航

首先是针对左右连接,这里与inner join区分

在使用left join时,on and 和on where会有区别

1. on的条件是在连接生成临时表时使用的条件,以左表为基准 ,不管on中的条件真否,都会返回左表中的记录
2.where条件是在临时表生成好后,再对临时表过滤。此时 和left join有区别(返回左表全部记录),条件不为真就全部过滤掉,on后的条件来生成左右表关联的临时表,
where后的条件是生成临时表后对临时表过滤

on and是进行韦恩运算时 连接时就做的动作,where是全部连接完后，再根据条件过滤

CREATE TABLE `a` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `sid` int(11) NOT NULL,
  `type` char(10) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

INSERT INTO `a` (`id`, `sid`, `type`) VALUES (1, 1, ‘a‘);
INSERT INTO `a` (`id`, `sid`, `type`) VALUES (2, 1, ‘b‘);
INSERT INTO `a` (`id`, `sid`, `type`) VALUES (3, 2, ‘c‘);
INSERT INTO `a` (`id`, `sid`, `type`) VALUES (4, 3, ‘d‘);

CREATE TABLE `b` (
  `sid` int(11) NOT NULL,
  `remark` char(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `b` (`sid`, `remark`) VALUES (1, ‘A‘);
INSERT INTO `b` (`sid`, `remark`) VALUES (2, ‘B‘);
INSERT INTO `b` (`sid`, `remark`) VALUES (3, ‘C‘);
INSERT INTO `b` (`sid`, `remark`) VALUES (4, ‘D‘);

mysql> select * from a;
+----+-----+------+
| id | sid | type |
+----+-----+------+
|  1 |   1 | a    |
|  2 |   1 | b    |
|  3 |   2 | c    |
|  4 |   3 | d    |
+----+-----+------+
4 rows in set

mysql> select * from b;
+-----+--------+
| sid | remark |
+-----+--------+
|   1 | A      |
|   2 | B      |
|   3 | C      |
|   4 | D      |
+-----+--------+
4 rows in set
mysql> select * from a left join b on a.sid=b.sid;
+----+-----+------+-----+--------+
| id | sid | type | sid | remark |
+----+-----+------+-----+--------+
|  1 |   1 | a    |   1 | A      |
|  2 |   1 | b    |   1 | A      |
|  3 |   2 | c    |   2 | B      |
|  4 |   3 | d    |   3 | C      |
+----+-----+------+-----+--------+
mysql> select * from a left join b on a.sid=b.sid and a.sid=1;
+----+-----+------+------+--------+
| id | sid | type | sid  | remark |
+----+-----+------+------+--------+
|  1 |   1 | a    |    1 | A      |
|  2 |   1 | b    |    1 | A      |
|  3 |   2 | c    | NULL | NULL   |
|  4 |   3 | d    | NULL | NULL   |
+----+-----+------+------+--------+
mysql> select * from a left join b on a.sid=b.sid where a.sid=1;
+----+-----+------+-----+--------+
| id | sid | type | sid | remark |
+----+-----+------+-----+--------+
|  1 |   1 | a    |   1 | A      |
|  2 |   1 | b    |   1 | A      |
+----+-----+------+-----+--------+

对于inner join

mysql> select * from a inner join b on a.sid=b.sid and a.sid=1;
+----+-----+------+-----+--------+
| id | sid | type | sid | remark |
+----+-----+------+-----+--------+
|  1 |   1 | a    |   1 | A      |
|  2 |   1 | b    |   1 | A      |
+----+-----+------+-----+--------+
mysql> select * from a inner join b on a.sid=b.sid where a.sid=1;
+----+-----+------+-----+--------+
| id | sid | type | sid | remark |
+----+-----+------+-----+--------+
|  1 |   1 | a    |   1 | A      |
|  2 |   1 | b    |   1 | A      |
+----+-----+------+-----+--------+

on and和on where结果一致
在使用inner join时，不管是对左表还是右表进行筛选，on and和on where都会对生成的临时表进行过滤