概念
- INNER JOIN（内连接）：获取两个表中字段匹配关系的记录。也就是只会返回共有的内容。
- LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
- RIGHT JOIN（右连接）： 获取右表所有记录，即使左表没有对应匹配的记录。
示例
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先在数据库中建立两张表student和score，具体内容如下：
【student】
mysql> select * from student; -------------- select * from student -------------- +----+---------------------+------+-------+------------+-----------+ | id | name | sex | birth | department | address | +----+---------------------+------+-------+------------+-----------+ | 1 | RooneyMara | F | 1985 | Psychology | American | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | | 3 | EllenPage | F | 1987 | Music | Canada | | 4 | TomHolland | M | 1996 | CS | England | | 5 | ScarlettJohansson | F | 1984 | Music | American | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | | 7 | EvaGreen | F | 1980 | Math | France | +----+---------------------+------+-------+------------+-----------+ 7 rows in set (0.00 sec)
【score】
mysql> select * from score; -------------- select * from score -------------- +----+--------+------------+-------+ | id | stu_id | c_name | grade | +----+--------+------------+-------+ | 1 | 1 | Psychology | 98 | | 2 | 1 | Music | 80 | | 3 | 2 | Psychology | 65 | | 4 | 2 | CS | 88 | | 5 | 3 | CS | 95 | | 6 | 4 | Psychology | 70 | | 7 | 4 | Music | 92 | | 8 | 5 | Music | 94 | | 9 | 6 | Psychology | 90 | | 10 | 6 | CS | 85 | | 11 | 8 | Music | 91 | +----+--------+------------+-------+ 11 rows in set (0.00 sec)
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内连接
查询student表中的所有个人信息及score表中的c_name,grade
mysql> select a.*,c_name,grade from student a join score b on a.id=b.stu_id; -------------- select a.*,c_name,grade from student a join score b on a.id=b.stu_id -------------- +----+---------------------+------+-------+------------+-----------+------------+-------+ | id | name | sex | birth | department | address | c_name | grade | +----+---------------------+------+-------+------------+-----------+------------+-------+ | 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 | | 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 | | 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 | | 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 | | 4 | TomHolland | M | 1996 | CS | England | Music | 92 | | 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 | +----+---------------------+------+-------+------------+-----------+------------+-------+ 10 rows in set (0.00 sec)
以上语句等价于：
mysql> select a.*,c_name,grade from student a,score b where a.id=b.stu_id; -------------- select a.*,c_name,grade from student a,score b where a.id=b.stu_id -------------- +----+---------------------+------+-------+------------+-----------+------------+-------+ | id | name | sex | birth | department | address | c_name | grade | +----+---------------------+------+-------+------------+-----------+------------+-------+ | 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 | | 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 | | 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 | | 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 | | 4 | TomHolland | M | 1996 | CS | England | Music | 92 | | 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 | +----+---------------------+------+-------+------------+-----------+------------+-------+ 10 rows in set (0.00 sec)
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左连接
student表中id为7的数据，在score中没有对应的内容。所以最后一条查询结果c_name,grade对应内容为null。
mysql> select a.*,c_name,grade from student a left join score b on a.id=b.stu_id; -------------- select a.*,c_name,grade from student a left join score b on a.id=b.stu_id -------------- +----+---------------------+------+-------+------------+-----------+------------+-------+ | id | name | sex | birth | department | address | c_name | grade | +----+---------------------+------+-------+------------+-----------+------------+-------+ | 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 | | 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 | | 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 | | 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 | | 4 | TomHolland | M | 1996 | CS | England | Music | 92 | | 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 | | 7 | EvaGreen | F | 1980 | Math | France | NULL | NULL | +----+---------------------+------+-------+------------+-----------+------------+-------+ 11 rows in set (0.00 sec)
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右连接
score表中id为11的数据，在student中没有对应的内容，所以最后一条查询结果id,name,sex等对应内容为null。
mysql> select a.*,c_name,grade from student a right join score b on a.id=b.stu_id; -------------- select a.*,c_name,grade from student a right join score b on a.id=b.stu_id -------------- +------+---------------------+------+-------+------------+-----------+------------+-------+ | id | name | sex | birth | department | address | c_name | grade | +------+---------------------+------+-------+------------+-----------+------------+-------+ | 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 | | 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 | | 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 | | 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 | | 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 | | 4 | TomHolland | M | 1996 | CS | England | Music | 92 | | 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 | | 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 | | NULL | NULL | NULL | NULL | NULL | NULL | Music | 91 | +------+---------------------+------+-------+------------+-----------+------------+-------+ 11 rows in set (0.00 sec)