mysql 内连接左连接右连接外连接

蔚落 2021-11-01 13:04 446阅读 0赞

mysql> desc student;  
\+-------+-------------+------+-----+---------+-------+  
| Field | Type | Null | Key | Default | Extra |  
\+-------+-------------+------+-----+---------+-------+  
| ID | int(11) | NO | PRI | 0 | |  
| NAME | varchar(16) | YES | | NULL | |  
| AGE | int(11) | YES | | NULL | |  
\+-------+-------------+------+-----+---------+-------+  
3 rows in set

mysql> desc sc;  
\+-------+---------+------+-----+---------+----------------+  
| Field | Type | Null | Key | Default | Extra |  
\+-------+---------+------+-----+---------+----------------+  
| ID | int(11) | NO | PRI | NULL | auto\_increment |  
| SID | int(11) | YES | | NULL | |  
| CID | int(11) | YES | MUL | NULL | |  
| SCORE | int(11) | YES | | NULL | |  
\+-------+---------+------+-----+---------+----------------+  
4 rows in set

mysql> select \* from student;  
\+-------+----------+-----+  
| ID | NAME | AGE |  
\+-------+----------+-----+  
| 10001 | Andy | 26 |  
| 10002 | Bill | 27 |  
| 10003 | Caroline | 34 |  
| 10004 | David | 46 |  
\+-------+----------+-----+  
4 rows in set

mysql> select \* from sc;  
\+----+-------+-----+-------+  
| ID | SID | CID | SCORE |  
\+----+-------+-----+-------+  
| 1 | 10001 | 101 | 78 |  
| 2 | 10001 | 102 | 67 |  
| 3 | 10008 | 103 | 100 |  
\+----+-------+-----+-------+  
3 rows in set  
\-----------------------------------------------------------------------------------------------------------------  
内连接  
mysql> select student.\*, sc.\* from student inner join sc on student.id = sc.sid;  
\+-------+------+-----+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+-----+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
\+-------+------+-----+----+-------+-----+-------+  
2 rows in set

内连接等价于我们平时的自然连接，也就是：  
mysql> select student.\*, sc.\* from student,sc where student.id = sc.sid;  
\+-------+------+-----+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+-----+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
\+-------+------+-----+----+-------+-----+-------+  
2 rows in set

\-----------------------------------------------------------------------------------------------------------------  
左连接  
考虑下面的需求，我想列出所有学生对应的成绩，一个学生可能多个成绩，也可能没有成绩。有多个成绩把多个成绩列出来，没有成绩的话，成绩这些字段的值使用NULL填充。怎么解决这个问题？  
使用左连接，student表 left join sc表，如下：  
mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid;  
\+-------+----------+-----+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+-----+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
\+-------+----------+-----+------+-------+------+-------+  
5 rows in set

\-----------------------------------------------------------------------------------------------------------------  
右连接  
考虑下面的需求，我想列出所有成绩对应的学生，一个成绩有对应的学生，也可能没有对应的学生，比如这个学生开除了。有学生就把学生列出来，没有学生的话，学生这些字段的值使用NULL填充。怎么解决这个问题？  
使用右连接，student表 right join sc表，如下：  
mysql> select student.\*, sc.\* from student right join sc on student.id = sc.sid;  
\+-------+------+------+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+------+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
| NULL | NULL | NULL | 3 | 10008 | 103 | 100 |  
\+-------+------+------+----+-------+-----+-------+  
3 rows in set

根据对称性，A left join B 等价于 B right join A  
\-----------------------------------------------------------------------------------------------------------------  
外连接  
左连接也叫左外连接（同理右连接），这里的外连接也叫全外连接。  
考虑下面的需求，我想列出所有学生对应的所有成绩，这里存在学生可能没有成绩，成绩也可能没有对应的学生，没有的话，也要列出来，这些字段的值使用NULL填充。  
目前，mysql不支持外连接，解决办法是使用union组合查询，把左连接和右连接的结果合并。如下：  
mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid union select student.\*, sc.\* from student right join sc on student.id = sc.sid;  
\+-------+----------+------+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+------+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
| NULL | NULL | NULL | 3 | 10008 | 103 | 100 |  
\+-------+----------+------+------+-------+------+-------+  
6 rows in set

注意：这里的union自动去除了重复行，如果不想去除重复行，使用union all

\-----------------------------------------------------------------------------------------------------------------  
还有一点需要注意：就是on 之后的条件，如下：  
mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid;  
\+-------+----------+-----+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+-----+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
\+-------+----------+-----+------+-------+------+-------+  
5 rows in set

mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid and sc.cid=101;  
\+-------+----------+-----+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+-----+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
\+-------+----------+-----+------+-------+------+-------+  
4 rows in set

mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid where sc.cid=101;  
\+-------+------+-----+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+-----+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
\+-------+------+-----+----+-------+-----+-------+  
1 row in set

这里看出第二个查询和第三个查询的区别，换一种写法就很清楚了。  
mysql> select student.\*, sc.\* from student left join sc on (student.id = sc.sid and sc.cid=101);  
\+-------+----------+-----+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+-----+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
\+-------+----------+-----+------+-------+------+-------+  
4 rows in set

mysql> select student.\*, sc.\* from student left join sc on (student.id = sc.sid) where sc.cid=101;  
\+-------+------+-----+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+-----+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
\+-------+------+-----+----+-------+-----+-------+  
1 row in set

mysql> select student.\*, sc.\* from student left join sc on (student.id = sc.sid where sc.cid=101);  
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where sc.cid=101)' at line 1

\-----------------------------------------------------------------------------------------------------------------  
select student.\*, sc.\* from student left join sc on (student.id = sc.sid and sc.cid=101); 相当于：  
1、内部连接  
mysql> select student.\*, sc.\* from student inner join sc on student.id = sc.sid;  
\+-------+------+-----+----+-------+-----+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+------+-----+----+-------+-----+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
\+-------+------+-----+----+-------+-----+-------+  
2 rows in set  
2、选择出sc.cid=101，再进行左连接，没有成绩的使用NULL填充

\-----------------------------------------------------------------------------------------------------------------  
select student.\*, sc.\* from student left join sc on (student.id = sc.sid) where sc.cid=101; 相当于：  
1、左连接  
mysql> select student.\*, sc.\* from student left join sc on student.id = sc.sid;  
\+-------+----------+-----+------+-------+------+-------+  
| ID | NAME | AGE | ID | SID | CID | SCORE |  
\+-------+----------+-----+------+-------+------+-------+  
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |  
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |  
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |  
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |  
| 10004 | David | 46 | NULL | NULL | NULL | NULL |  
\+-------+----------+-----+------+-------+------+-------+  
5 rows in set  
2、再选出 sc.cid=101

转载于:https://www.cnblogs.com/nzbbody/p/4572705.html