mysql中的JOIN用法总结
join是mysql中一个基础的关键词，一般在多表连接查询中使用，这里做一下总结
1、JOIN的语法格式
table_references: table_reference [, table_reference] ... table_reference: table_factor | join_table table_factor: tbl_name [[AS] alias] [{USE|IGNORE|FORCE} INDEX (key_list)] | ( table_references ) | { OJ table_reference LEFT OUTER JOIN table_reference ON conditional_expr } join_table: table_reference [INNER | CROSS] JOIN table_factor [join_condition] | table_reference STRAIGHT_JOIN table_factor | table_reference STRAIGHT_JOIN table_factor ON condition | table_reference LEFT [OUTER] JOIN table_reference join_condition | table_reference NATURAL [LEFT [OUTER]] JOIN table_factor | table_reference RIGHT [OUTER] JOIN table_reference join_condition | table_reference NATURAL [RIGHT [OUTER]] JOIN table_factor join_condition: ON conditional_expr | USING (column_list)
2、JOIN解析说明
我们先准备实验例子（mysql 版本：mysql Ver 14.12 Distrib 5.0.95, for redhat-linux-gnu (i686) using readline 5.1）
CREATE TABLE `test_join_a` ( `id` bigint(20) NOT NULL DEFAULT '0', `name` varchar(200), PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE `test_join_b` ( `id` bigint(20) NOT NULL DEFAULT '0', `sex` varchar(200), PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE `test_join_c` ( `id` bigint(20) NOT NULL DEFAULT '0', `class` varchar(200), PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; insert into test_join_a (id) values (1); insert into test_join_a (id, name) values (2, "abc"); insert into test_join_a (id, name) values (3, "abcd"); insert into test_join_a (id, name) values (4, "abcde"); insert into test_join_b (id) values (1); insert into test_join_b (id, sex) values (2, "abc"); insert into test_join_b (id, sex) values (3, "abc"); insert into test_join_c (id) values (1); insert into test_join_c (id, class) values (2, "abc"); insert into test_join_c (id, class) values (3, "abc");
a、关于LEFT JOIN
对于 A LEFT JOIN (B， C) ON join_condition ，A表m，B表n， C表t 条记录 【说明：由于未发现专业术语描述A，B，C三表区别，此处，将A表暂称基础表，B,C表称附加表】
* 以A表为基础表，从B表和C表中挑选符合ON join_condition条件的数据，如果没有符合的，则附加表(B,C)的列对应的行设置为NULL。
* 通过这种连接方式，查询后的数据条数，最多为 m * n * t条， 最少为 m条
同理，对于(A, B) LEFT JOIN C ON join_condition
* 以A， B表为基础表，从C表中挑选符合ON join_condition条件的数据，如果没有符合的，则附加表(C)的列对应的行设置为NULL。
* 通过这种连接方式，查询后的数据条数，最多为 m * n * t条， 最少为 m * n条
在LEFT JOIN 和 RIGHT JOIN 中，ON 子句后还有 WHERE 子句的筛选条件，区别在于
1、where子句可以省略，ON 子句不能省略，如果不用ON join_condition，则可以使用ON 1=1 或者 ON 1
2、ON join_condtion 会以基础表的笛卡尔积为基础，附加表没有符合ON join_condtion的记录，则将附加表列对应的行设为NULL。
3、where 子句会将 ON join_condition生成的虚拟表做筛选。所以通过where子句后，获取的记录数，最少为 0条
注意： NULL=NULL 既不符合 join的on condition 也不符合 where子句的筛选
下面是两个实验过程，a表4条记录，b表3条，c表3条：
1、可以看到生成记录条数4 * 3 * 3 = 36 条
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) on 1=1; +-----+-------+------+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+-------+------+------+------+-------+ | 1 | NULL | 1 | NULL | 1 | NULL | | 1 | NULL | 1 | NULL | 2 | abc | | 1 | NULL | 1 | NULL | 3 | abc | | 1 | NULL | 2 | abc | 1 | NULL | | 1 | NULL | 2 | abc | 2 | abc | | 1 | NULL | 2 | abc | 3 | abc | | 1 | NULL | 3 | abc | 1 | NULL | | 1 | NULL | 3 | abc | 2 | abc | | 1 | NULL | 3 | abc | 3 | abc | | 2 | abc | 1 | NULL | 1 | NULL | | 2 | abc | 1 | NULL | 2 | abc | | 2 | abc | 1 | NULL | 3 | abc | | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 3 | abc | 3 | abc | | 3 | abcd | 1 | NULL | 1 | NULL | | 3 | abcd | 1 | NULL | 2 | abc | | 3 | abcd | 1 | NULL | 3 | abc | | 3 | abcd | 2 | abc | 1 | NULL | | 3 | abcd | 2 | abc | 2 | abc | | 3 | abcd | 2 | abc | 3 | abc | | 3 | abcd | 3 | abc | 1 | NULL | | 3 | abcd | 3 | abc | 2 | abc | | 3 | abcd | 3 | abc | 3 | abc | | 4 | abcde | 1 | NULL | 1 | NULL | | 4 | abcde | 1 | NULL | 2 | abc | | 4 | abcde | 1 | NULL | 3 | abc | | 4 | abcde | 2 | abc | 1 | NULL | | 4 | abcde | 2 | abc | 2 | abc | | 4 | abcde | 2 | abc | 3 | abc | | 4 | abcde | 3 | abc | 1 | NULL | | 4 | abcde | 3 | abc | 2 | abc | | 4 | abcde | 3 | abc | 3 | abc | +-----+-------+------+------+------+-------+ 36 rows in set (0.00 sec)
2、筛选符合条件的记录，如果没有符合的，则设置为NULL，参看实验表中aid=1，3, 4情况。
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) on
a.name=b.sex; +-----+-------+------+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+-------+------+------+------+-------+ | 1 | NULL | NULL | NULL | NULL | NULL | | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 3 | abc | 3 | abc | | 3 | abcd | NULL | NULL | NULL | NULL | | 4 | abcde | NULL | NULL | NULL | NULL | +-----+-------+------+------+------+-------+ 9 rows in set (0.00 sec)
3、针对 (A, B) LEFT JOIN C ON join_condition 的情况，筛选符合条件的记录，以A，B表的笛卡尔积为基础，查找C中符合 on join_condition的记录数，如果没有，则将C中对应列的行值设置未NULL。
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a, test_join_b b) left join test_join_c c on b.id>5; +-----+-------+-----+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+-------+-----+------+------+-------+ | 1 | NULL | 1 | NULL | NULL | NULL | | 1 | NULL | 2 | abc | NULL | NULL | | 1 | NULL | 3 | abc | NULL | NULL | | 2 | abc | 1 | NULL | NULL | NULL | | 2 | abc | 2 | abc | NULL | NULL | | 2 | abc | 3 | abc | NULL | NULL | | 3 | abcd | 1 | NULL | NULL | NULL | | 3 | abcd | 2 | abc | NULL | NULL | | 3 | abcd | 3 | abc | NULL | NULL | | 4 | abcde | 1 | NULL | NULL | NULL | | 4 | abcde | 2 | abc | NULL | NULL | | 4 | abcde | 3 | abc | NULL | NULL | +-----+-------+-----+------+------+-------+ 12 rows in set (0.00 sec)
4、可以看出ON condition的执行在WHERE之前，ON condition会形成虚拟表。
mysql> explain select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) on b.id>5 where b.id>5; +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------------+ | 1 | SIMPLE | b | range | PRIMARY | PRIMARY | 8 | NULL | 1 | Using where | | 1 | SIMPLE | c | ALL | NULL | NULL | NULL | NULL | 3 | Using join buffer | | 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL | 4 | Using join buffer | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------------+ 3 rows in set (0.00 sec)
5、WHERE 对虚拟表的筛选和ON条件是不一样的，ON condition会保存基础表的每条数据，附加表中没有符合的，则将附加表列设置为NULL，WHERE 只是筛选符合WHERE 子句的记录。 (可以从下面的第三次查询的结果看出，查询的是通过ON condition形成的虚拟表)
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) on b.id>5; +-----+-------+------+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+-------+------+------+------+-------+ | 1 | NULL | NULL | NULL | NULL | NULL | | 2 | abc | NULL | NULL | NULL | NULL | | 3 | abcd | NULL | NULL | NULL | NULL | | 4 | abcde | NULL | NULL | NULL | NULL | +-----+-------+------+------+------+-------+ 4 rows in set (0.00 sec) mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) on b.id>5 where b.id>5; Empty set (0.00 sec)
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) ON a.name=b.sex where b.sex is NULL;
+-----+-------+------+------+------+-------+
| aid | name | bid | sex | cid | class |
+-----+-------+------+------+------+-------+
| 1 | NULL | NULL | NULL | NULL | NULL |
| 3 | abcd | NULL | NULL | NULL | NULL |
| 4 | abcde | NULL | NULL | NULL | NULL |
+-----+-------+------+------+------+-------+
6、NULL=NULL 既不符合 join的on condition 也不符合 where子句的筛选。
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) ON a.name=b.sex; +-----+-------+------+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+-------+------+------+------+-------+ | 1 | NULL | NULL | NULL | NULL | NULL | | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 3 | abc | 3 | abc | | 3 | abcd | NULL | NULL | NULL | NULL | | 4 | abcde | NULL | NULL | NULL | NULL | +-----+-------+------+------+------+-------+ 9 rows in set (0.00 sec) mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from test_join_a a left join (test_join_b b, test_join_c c) ON a.name=b.sex where a.name=b.sex; +-----+------+------+------+------+-------+ | aid | name | bid | sex | cid | class | +-----+------+------+------+------+-------+ | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 3 | abc | +-----+------+------+------+------+-------+ 6 rows in set (0.00 sec)
b、关于RIGHT JOIN
right join和 left join类似，区别在于
1、对于 A right join (B, C) on join_condition ，会以 B,C表为基础表，A表为附加表
2、对于 (B, C) right join A on join_condition ，会以 A 表为基础表，B,C表为附加表
C、关于JOIN
JOIN与LEFT JOIN 和RIGHT JOIN的区别在于 JOIN 不区别左右基础表，可以认为他的ON condition 退化成where 子句形式，
例如：对于下面的SQL
select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON a.name=b.sex;
结果上等价于
select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON 1=1 where a.name=b.sex;
同时对于 A JOIN (B, C) 等价于 （A，B) JOIN C
关于JOIN的解析过程，我们可以理解成， 将A,B,C表做笛卡尔积，构成新的虚拟表， ON condition转化成where 子句，结合后面的where 子句做筛选，得出筛选结果。
我们来看一下实验情况：
mysql> explain select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON a.name=b.sex; +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | 1 | SIMPLE | b | ALL | NULL | NULL | NULL | NULL | 3 | | | 1 | SIMPLE | c | ALL | NULL | NULL | NULL | NULL | 3 | Using join buffer | | 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL | 4 | Using where; Using join buffer | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ 3 rows in set (0.00 sec) mysql> explain select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON 1=1 where a.name=b.sex; +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | 1 | SIMPLE | b | ALL | NULL | NULL | NULL | NULL | 3 | | | 1 | SIMPLE | c | ALL | NULL | NULL | NULL | NULL | 3 | Using join buffer | | 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL | 4 | Using where; Using join buffer | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ 3 rows in set (0.00 sec)
可以看出，两个sql的explain结果是一样的，拥有相同的执行计划
我们再看看这两个sql的执行结果
mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON a.name=b.sex; +-----+------+-----+------+-----+-------+ | aid | name | bid | sex | cid | class | +-----+------+-----+------+-----+-------+ | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 3 | abc | +-----+------+-----+------+-----+-------+ 6 rows in set (0.00 sec) mysql> select a.id as aid, a.name, b.id as bid, b.sex, c.id as cid, c.class from (test_join_a a ,test_join_b b) join test_join_c c ON 1=1 where a.name=b.sex; +-----+------+-----+------+-----+-------+ | aid | name | bid | sex | cid | class | +-----+------+-----+------+-----+-------+ | 2 | abc | 2 | abc | 1 | NULL | | 2 | abc | 3 | abc | 1 | NULL | | 2 | abc | 2 | abc | 2 | abc | | 2 | abc | 3 | abc | 2 | abc | | 2 | abc | 2 | abc | 3 | abc | | 2 | abc | 3 | abc | 3 | abc | +-----+------+-----+------+-----+-------+ 6 rows in set (0.00 sec)
