【數據庫】MySQL連接-內連接、外連接、交叉連接

MySQL中各種連接操作

以person和address兩張表為例進行說明，兩表結構如下：

select * from person;
+----+------+------+
| id | name | age  |
+----+------+------+
|  1 | AAA  |   18 |
|  2 | BBB  |   25 |
|  3 | CCC  |   21 |
+----+------+------+
3 rows in set (0.00 sec)

select * from address;
+----+----------+
| id | address  |
+----+----------+
|  1 | HOME_AAA |
|  2 | HOME_BBB |
|  4 | HOME_DDD |
+----+----------+
3 rows in set (0.08 sec)

1.內連接

1.1 自然連接（natural join）

自然連接只考慮兩個關系中在共同屬性上取值相同的元組對。結果中無null，不需要使用修飾詞限制連接屬性。
在本例中，共同屬性為id，結果如下：

select * from person natural join address;
+----+------+------+----------+
| id | name | age  | address  |
+----+------+------+----------+
|  1 | AAA  |   18 | HOME_AAA |
|  2 | BBB  |   25 | HOME_BBB |
+----+------+------+----------+
2 rows in set (0.05 sec)

1.2 內連接（inner join）

默認情況下inner join和join兩種語法等價。
對于內連接，必須使用using或on指定連接屬性/條件，否則產生的結果與交叉連接相同（結果均為兩個表的笛卡爾積），在后文中介紹。

使用using指定連接屬性，結果中只包含1個id屬性：
select * from person join address using(id);
+----+------+------+----------+
| id | name | age  | address  |
+----+------+------+----------+
|  1 | AAA  |   18 | HOME_AAA |
|  2 | BBB  |   25 | HOME_BBB |
+----+------+------+----------+
2 rows in set (0.00 sec)
使用on指定連接條件，結果中包含2個id屬性：
select * from person inner join address on person.id = address.id;
+----+------+------+----+----------+
| id | name | age  | id | address  |
+----+------+------+----+----------+
|  1 | AAA  |   18 |  1 | HOME_AAA |
|  2 | BBB  |   25 |  2 | HOME_BBB |
+----+------+------+----+----------+
2 rows in set (0.00 sec)

2.外連接

2.1 左外連接（left outer join）

默認情況下left outer join和left join兩種語法等價。
以左邊的表為主，在右邊的表中找到所有滿足條件的元素，并把他們連接起來，如果沒有對應的元素，則在相應位置上的值為null。
特別注意：使用外連接必須使用using或on指定連接屬性或連接條件，否則會報錯。

使用using指定連接屬性，結果中只包含1個id屬性：
select * from person left outer join address using(id);
+----+------+------+----------+
| id | name | age  | address  |
+----+------+------+----------+
|  1 | AAA  |   18 | HOME_AAA |
|  2 | BBB  |   25 | HOME_BBB |
|  3 | CCC  |   21 | NULL     |
+----+------+------+----------+
3 rows in set (0.00 sec)
使用on指定連接條件，結果中包含2個id屬性：
select * from person left outer join address on person.id = address.id;
+----+------+------+------+----------+
| id | name | age  | id   | address  |
+----+------+------+------+----------+
|  1 | AAA  |   18 |    1 | HOME_AAA |
|  2 | BBB  |   25 |    2 | HOME_BBB |
|  3 | CCC  |   21 | NULL | NULL     |
+----+------+------+------+----------+
3 rows in set (0.00 sec)

2.2 右外連接（right outer join）

默認情況下right outer join和right join兩種語法等價。
與左外連接相反，以右邊的表為主，在左邊的表中找到所有滿足條件的元素，并把他們連接起來，如果沒有對應的元素，則在相應位置上的值為null。
特別注意：使用外連接必須使用using或on指定連接屬性或連接條件，否則會報錯。

使用using指定連接屬性，結果中只包含1個id屬性：
select * from person right outer join address using(id);
+----+----------+------+------+
| id | address  | name | age  |
+----+----------+------+------+
|  1 | HOME_AAA | AAA  |   18 |
|  2 | HOME_BBB | BBB  |   25 |
|  4 | HOME_DDD | NULL | NULL |
+----+----------+------+------+
3 rows in set (0.04 sec)
使用on指定連接條件，結果中包含2個id屬性：
select * from person right outer join address on person.id = address.id;
+------+------+------+----+----------+
| id   | name | age  | id | address  |
+------+------+------+----+----------+
|    1 | AAA  |   18 |  1 | HOME_AAA |
|    2 | BBB  |   25 |  2 | HOME_BBB |
| NULL | NULL | NULL |  4 | HOME_DDD |
+------+------+------+----+----------+
3 rows in set (0.02 sec)

2.3 全外連接（full outer join）

全外連接顯示所有表的行、列，條件不匹配的值為皆為null。
特別注意：在MySQL中不支持全外連接操作，可通過Union左外連接和右外連接來實現。

使用on指定連接條件，結果中包含2個id屬性：
mysql> select * from person left outer join address on person.id = address.id
    -> union
    -> select * from person right outer join address on person.id = address.id;
+------+------+------+------+----------+
| id   | name | age  | id   | address  |
+------+------+------+------+----------+
|    1 | AAA  |   18 |    1 | HOME_AAA |
|    2 | BBB  |   25 |    2 | HOME_BBB |
|    3 | CCC  |   21 | NULL | NULL     |
| NULL | NULL | NULL |    4 | HOME_DDD |
+------+------+------+------+----------+
4 rows in set (0.40 sec)

3.交叉連接（cross join）

交叉連接所返回結果即為笛卡爾積，即：對于兩個不同的集合A和B，對于A中的每一個元素，都有對于在B中的所有元素做連接運算。因此對于兩個元組分別為m，n的表，笛卡爾積后得到的元組個數為m x n。
由于交叉連接與笛卡爾積結果相同，因此如下兩種寫法相同：

寫法1：交叉連接cross join 
select * from person cross join address;
+----+------+------+----+----------+
| id | name | age  | id | address  |
+----+------+------+----+----------+
|  1 | AAA  |   18 |  1 | HOME_AAA |
|  2 | BBB  |   25 |  1 | HOME_AAA |
|  3 | CCC  |   21 |  1 | HOME_AAA |
|  1 | AAA  |   18 |  2 | HOME_BBB |
|  2 | BBB  |   25 |  2 | HOME_BBB |
|  3 | CCC  |   21 |  2 | HOME_BBB |
|  1 | AAA  |   18 |  4 | HOME_DDD |
|  2 | BBB  |   25 |  4 | HOME_DDD |
|  3 | CCC  |   21 |  4 | HOME_DDD |
+----+------+------+----+----------+
9 rows in set (0.00 sec)
寫法2：笛卡爾積
select * from person,address;
+----+------+------+----+----------+
| id | name | age  | id | address  |
+----+------+------+----+----------+
|  1 | AAA  |   18 |  1 | HOME_AAA |
|  2 | BBB  |   25 |  1 | HOME_AAA |
|  3 | CCC  |   21 |  1 | HOME_AAA |
|  1 | AAA  |   18 |  2 | HOME_BBB |
|  2 | BBB  |   25 |  2 | HOME_BBB |
|  3 | CCC  |   21 |  2 | HOME_BBB |
|  1 | AAA  |   18 |  4 | HOME_DDD |
|  2 | BBB  |   25 |  4 | HOME_DDD |
|  3 | CCC  |   21 |  4 | HOME_DDD |
+----+------+------+----+----------+
9 rows in set (0.00 sec)
當使用內鏈接不加連接屬性或條件時，結果也為笛卡爾積：
select * from person inner join address;
+----+------+------+----+----------+
| id | name | age  | id | address  |
+----+------+------+----+----------+
|  1 | AAA  |   18 |  1 | HOME_AAA |
|  2 | BBB  |   25 |  1 | HOME_AAA |
|  3 | CCC  |   21 |  1 | HOME_AAA |
|  1 | AAA  |   18 |  2 | HOME_BBB |
|  2 | BBB  |   25 |  2 | HOME_BBB |
|  3 | CCC  |   21 |  2 | HOME_BBB |
|  1 | AAA  |   18 |  4 | HOME_DDD |
|  2 | BBB  |   25 |  4 | HOME_DDD |
|  3 | CCC  |   21 |  4 | HOME_DDD |
+----+------+------+----+----------+
9 rows in set (0.09 sec)

4.連接語句中on與where的區別

on和where的查詢順序不同。
標準查詢關鍵字執行順序為：from > where > group by > having > order by
在連接語句中，join是在from范圍內，而on條件是對join進行條件過濾，所以會先on條件篩選表，然后再做join；對于where來說，是在join執行后生成的臨時表上再進行條件過濾。
以right join的on條件和where條件進行舉例說明：

例1：on條件中增加對左表的限制條件

結果：返回右表的全部記錄，左表的篩選條件生效

select * from person right join address on person.id = address.id and person.age<20;
+------+------+------+----+----------+
| id   | name | age  | id | address  |
+------+------+------+----+----------+
|    1 | AAA  |   18 |  1 | HOME_AAA |
| NULL | NULL | NULL |  2 | HOME_BBB |
| NULL | NULL | NULL |  4 | HOME_DDD |
+------+------+------+----+----------+
3 rows in set (0.00 sec)
例2：on條件后增加where限制條件

結果：where后邊的條件是對最終的臨時表進行篩選，on后邊的條件是作為匹配條件進行篩選

select * from person right join address on person.id = address.id where person.age<20;
+------+------+------+----+----------+
| id   | name | age  | id | address  |
+------+------+------+----+----------+
|    1 | AAA  |   18 |  1 | HOME_AAA |
+------+------+------+----+----------+
1 row in set (0.03 sec)

附錄：SQL連接方式圖片說明

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