MySQL中join用法解析
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MySQL中join用法解析
实例数据库如下:
student表:
mysql> select * from student;+-----------+-----------+------+------+-------+| Sno | Sname | Ssex | Sage | Sdept |+-----------+-----------+------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS || 201215122 | 刘晨 | 女 | 19 | CS || 201215123 | 王敏 | 女 | 18 | MA || 201215125 | 张立 | 男 | 19 | IS || 201215128 | 陈冬 | 男 | 18 | IS || 201215126 | 张成民 | 男 | 18 | CS |+-----------+-----------+------+------+-------+6 rows in set (0.00 sec)
sc表:
mysql> select * from sc;+-----------+------+-------+| Sno | Cno | Grade |+-----------+------+-------+| 201215121 | 1 | 92 || 201215121 | 2 | 85 || 201215121 | 3 | 88 || 201215122 | 2 | 90 || 201215122 | 3 | 80 || 201215128 | 1 | 78 |+-----------+------+-------+6 rows in set (0.00 sec)
LEFT JOIN(左连接)
sql语句如下:
select * from student left join sc on student.Sno=sc.Sno;
运行结果如下:
+-----------+-----------+------+------+-------+-----------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade |+-----------+-----------+------+------+-------+-----------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL |+-----------+-----------+------+------+-------+-----------+------+-------+
在此例中left join 是以student表中的记录为基础,student表可以看成左表,sc表可以看成右表,左表中的记录会完全显示出来,加上匹配到的右表,如果左边没有匹配到,则其余部分显示为null。
USING字句
using字句和on字句,类似,但结果略有不同。
例如:
mysql> select student.Sno,Sname,Grade from student left join sc on student.Sno=sc.Sno;+-----------+-----------+-------+| Sno | Sname | Grade |+-----------+-----------+-------+| 201215121 | 李勇 | 92 || 201215121 | 李勇 | 85 || 201215121 | 李勇 | 88 || 201215122 | 刘晨 | 90 || 201215122 | 刘晨 | 80 || 201215128 | 陈冬 | 78 || 201215123 | 王敏 | NULL || 201215125 | 张立 | NULL || 201215126 | 张成民 | NULL |+-----------+-----------+-------+9 rows in set (0.00 sec)
以上等价于
select Sno,Sname,Grade from student left join sc using(Sno);+-----------+-----------+-------+| Sno | Sname | Grade |+-----------+-----------+-------+| 201215121 | 李勇 | 92 || 201215121 | 李勇 | 85 || 201215121 | 李勇 | 88 || 201215122 | 刘晨 | 90 || 201215122 | 刘晨 | 80 || 201215128 | 陈冬 | 78 || 201215123 | 王敏 | NULL || 201215125 | 张立 | NULL || 201215126 | 张成民 | NULL |+-----------+-----------+-------+
不同的地方,例如:
select * from student left join sc on student.Sno=sc.Sno;+-----------+-----------+------+------+-------+-----------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade |+-----------+-----------+------+------+-------+-----------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL |+-----------+-----------+------+------+-------+-----------+------+-------+
select * from student left join sc using (sno);+-----------+-----------+------+------+-------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Cno | Grade |+-----------+-----------+------+------+-------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL |+-----------+-----------+------+------+-------+------+-------+
重复的Sno列,如果用on字句会被输出两次
RIGHT JOIN(右连接)
同LEFT JOIN,只不过以右表为基础,例如:
select * from student right join sc using (sno);+-----------+------+-------+--------+------+------+-------+| Sno | Cno | Grade | Sname | Ssex | Sage | Sdept |+-----------+------+-------+--------+------+------+-------+| 201215121 | 1 | 92 | 李勇 | 男 | 22 | CS || 201215121 | 2 | 85 | 李勇 | 男 | 22 | CS || 201215121 | 3 | 88 | 李勇 | 男 | 22 | CS || 201215122 | 2 | 90 | 刘晨 | 女 | 19 | CS || 201215122 | 3 | 80 | 刘晨 | 女 | 19 | CS || 201215128 | 1 | 78 | 陈冬 | 男 | 18 | IS |+-----------+------+-------+--------+------+------+-------+
INNER JOIN(相等连接或内连接)
不会显示以谁为基础,只会显示符合条件的记录
select * from student inner join sc on student.Sno=sc.Sno;+-----------+--------+------+------+-------+-----------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade |+-----------+--------+------+------+-------+-----------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 |+-----------+--------+------+------+-------+-----------+------+-------+
以上语句等同于:
select * from student,sc where student.Sno=sc.Sno;+-----------+--------+------+------+-------+-----------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade |+-----------+--------+------+------+-------+-----------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 |+-----------+--------+------+------+-------+-----------+------+-------+
扩展
如果只想从A表中取出一些记录,但不包含B表
可以在left join 后面加上一个where语句
select * from student left join sc using(Sno) where sc.Sno is null;+-----------+-----------+------+------+-------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Cno | Grade |+-----------+-----------+------+------+-------+------+-------+| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL |+-----------+-----------+------+------+-------+------+-------+
求差集
可以结合union字句,由于本例中,右侧的已经全部对应的所以显示结果,和上一个一致。
select * from student left join sc using(Sno) where student.Sno is null union select * from student left join sc using(Sno) where sc.Sno is null;+-----------+-----------+------+------+-------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Cno | Grade |+-----------+-----------+------+------+-------+------+-------+| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL |+-----------+-----------+------+------+-------+------+-------+
FULL JOIN
select * from student left join sc on student.Sno=sc.Sno union select * from student right join sc on student.Sno=sc.Sno;+-----------+-----------+------+------+-------+-----------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade |+-----------+-----------+------+------+-------+-----------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL |+-----------+-----------+------+------+-------+-----------+------+-------+
注:A left join B 等同于 B right join A
mysql> select * from student left join sc using(Sno);+-----------+-----------+------+------+-------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Cno | Grade |+-----------+-----------+------+------+-------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL |+-----------+-----------+------+------+-------+------+-------+9 rows in set (0.00 sec)mysql> select * from sc right join student using(Sno);+-----------+-----------+------+------+-------+------+-------+| Sno | Sname | Ssex | Sage | Sdept | Cno | Grade |+-----------+-----------+------+------+-------+------+-------+| 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 || 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 || 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 || 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 || 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 || 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 || 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL || 201215125 | 张立 | 男 | 19 | IS | NULL | NULL || 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL |+-----------+-----------+------+------+-------+------+-------+
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