mysql 內串連 左串連 右串連 外串連

標籤：

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
-----------------------------------------------------------------------------------------------------------------
內串連
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

內串連等價於我們平時的自然串連，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

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左串連
考慮下面的需求，我想列出所有學生對應的成績，一個學生可能多個成績，也可能沒有成績。有多個成績把多個成績列出來，沒有成績的話，成績這些欄位的值使用NULL填充。怎麼解決這個問題？
使用左串連，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

-----------------------------------------------------------------------------------------------------------------
右串連
考慮下面的需求，我想列出所有成績對應的學生，一個成績有對應的學生，也可能沒有對應的學生，比如這個學生開除了。有學生就把學生列出來，沒有學生的話，學生這些欄位的值使用NULL填充。怎麼解決這個問題？
使用右串連，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

根據對稱性，A left join B 等價於 B right join A
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外串連
左串連也叫左外串連（同理右串連），這裡的外串連也叫全外串連。
考慮下面的需求，我想列出所有學生對應的所有成績，這裡存在學生可能沒有成績，成績也可能沒有對應的學生，沒有的話，也要列出來，這些欄位的值使用NULL填充。
目前，mysql不支援外串連，解決辦法是使用union組合查詢，把左串連和右串連的結果合并。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：這裡的union自動去除了重複行，如果不想去除重複行，使用union all

-----------------------------------------------------------------------------------------------------------------
還有一點需要注意：就是on 之後的條件，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

這裡看出第二個查詢和第三個查詢的區別，換一種寫法就很清楚了。
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); 相當於：
1、內部串連
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、選擇出sc.cid=101，再進行左串連，沒有成績的使用NULL填充

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select student.*, sc.* from student left join sc on (student.id = sc.sid) where sc.cid=101; 相當於：
1、左串連
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、再選出 sc.cid=101

mysql 內串連 左串連 右串連 外串連