JOIN的含义就如英文单词“join”一样,连接两张表,大致分为内连接,外连接,右连接,左连接,自然连接。这里描述先甩出一张用烂了的图,然后插入测试数据。

复制代码
CREATE TABLE t_blog(
id INT PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(50),
typeId INT
);
SELECT FROM t_blog;
+----+-------+--------+
| id | title | typeId |
+----+-------+--------+
| 1 | aaa | 1 |
| 2 | bbb | 2 |
| 3 | ccc | 3 |
| 4 | ddd | 4 |
| 5 | eee | 4 |
| 6 | fff | 3 |
| 7 | ggg | 2 |
| 8 | hhh | NULL |
| 9 | iii | NULL |
| 10 | jjj | NULL |
+----+-------+--------+
-- 博客的类别
CREATE TABLE t_type(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
SELECT
FROM t_type;
+----+------------+
| id | name |
+----+------------+
| 1 | C++ |
| 2 | C |
| 3 | Java |
| 4 | C# |
| 5 | Javascript |
+----+------------+
复制代码
笛卡尔积:CROSS JOIN

要理解各种JOIN首先要理解笛卡尔积。笛卡尔积就是将A表的每一条记录与B表的每一条记录强行拼在一起。所以,如果A表有n条记录,B表有m条记录,笛卡尔积产生的结果就会产生n*m条记录。下面的例子,t_blog有10条记录,t_type有5条记录,所有他们俩的笛卡尔积有50条记录。有五种产生笛卡尔积的方式如下。

复制代码
SELECT FROM t_blog CROSS JOIN t_type;
SELECT
FROM t_blog INNER JOIN t_type;
SELECT FROM t_blog,t_type;
SELECT
FROM t_blog NATURE JOIN t_type;
select * from t_blog NATURA join t_type;
+----+-------+--------+----+------------+
| id | title | typeId | id | name |
+----+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | C++ |
| 1 | aaa | 1 | 2 | C |
| 1 | aaa | 1 | 3 | Java |
| 1 | aaa | 1 | 4 | C# |
| 1 | aaa | 1 | 5 | Javascript |
| 2 | bbb | 2 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 2 | bbb | 2 | 3 | Java |
| 2 | bbb | 2 | 4 | C# |
| 2 | bbb | 2 | 5 | Javascript |
| 3 | ccc | 3 | 1 | C++ |
| 3 | ccc | 3 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 3 | ccc | 3 | 4 | C# |
| 3 | ccc | 3 | 5 | Javascript |
| 4 | ddd | 4 | 1 | C++ |
| 4 | ddd | 4 | 2 | C |
| 4 | ddd | 4 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 4 | ddd | 4 | 5 | Javascript |
| 5 | eee | 4 | 1 | C++ |
| 5 | eee | 4 | 2 | C |
| 5 | eee | 4 | 3 | Java |
| 5 | eee | 4 | 4 | C# |
| 5 | eee | 4 | 5 | Javascript |
| 6 | fff | 3 | 1 | C++ |
| 6 | fff | 3 | 2 | C |
| 6 | fff | 3 | 3 | Java |
| 6 | fff | 3 | 4 | C# |
| 6 | fff | 3 | 5 | Javascript |
| 7 | ggg | 2 | 1 | C++ |
| 7 | ggg | 2 | 2 | C |
| 7 | ggg | 2 | 3 | Java |
| 7 | ggg | 2 | 4 | C# |
| 7 | ggg | 2 | 5 | Javascript |
| 8 | hhh | NULL | 1 | C++ |
| 8 | hhh | NULL | 2 | C |
| 8 | hhh | NULL | 3 | Java |
| 8 | hhh | NULL | 4 | C# |
| 8 | hhh | NULL | 5 | Javascript |
| 9 | iii | NULL | 1 | C++ |
| 9 | iii | NULL | 2 | C |
| 9 | iii | NULL | 3 | Java |
| 9 | iii | NULL | 4 | C# |
| 9 | iii | NULL | 5 | Javascript |
| 10 | jjj | NULL | 1 | C++ |
| 10 | jjj | NULL | 2 | C |
| 10 | jjj | NULL | 3 | Java |
| 10 | jjj | NULL | 4 | C# |
| 10 | jjj | NULL | 5 | Javascript |
+----+-------+--------+----+------------+
复制代码
内连接:INNER JOIN

内连接INNER JOIN是最常用的连接操作。从数学的角度讲就是求两个表的交集,从笛卡尔积的角度讲就是从笛卡尔积中挑出ON子句条件成立的记录。有INNER JOIN,WHERE(等值连接),STRAIGHT_JOIN,JOIN(省略INNER)四种写法。至于哪种好我会在MySQL的JOIN(二):优化讲述。示例如下。

复制代码
SELECT FROM t_blog INNER JOIN t_type ON t_blog.typeId=t_type.id;
SELECT
FROM t_blog,t_type WHERE t_blog.typeId=t_type.id;
SELECT FROM t_blog STRAIGHT_JOIN t_type ON t_blog.typeId=t_type.id; --注意STRIGHT_JOIN有个下划线
SELECT
FROM t_blog JOIN t_type ON t_blog.typeId=t_type.id;

+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
|  1 | aaa   |      1 |  1 | C++  |
|  2 | bbb   |      2 |  2 | C    |
|  7 | ggg   |      2 |  2 | C    |
|  3 | ccc   |      3 |  3 | Java |
|  6 | fff   |      3 |  3 | Java |
|  4 | ddd   |      4 |  4 | C#   |
|  5 | eee   |      4 |  4 | C#   |
+----+-------+--------+----+------+

复制代码

左连接:LEFT JOIN

左连接LEFT JOIN的含义就是求两个表的交集外加左表剩下的数据。依旧从笛卡尔积的角度讲,就是先从笛卡尔积中挑出ON子句条件成立的记录,然后加上左表中剩余的记录(见最后三条)。

复制代码
SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id;

+----+-------+--------+------+------+
| id | title | typeId | id   | name |
+----+-------+--------+------+------+
|  1 | aaa   |      1 |    1 | C++  |
|  2 | bbb   |      2 |    2 | C    |
|  7 | ggg   |      2 |    2 | C    |
|  3 | ccc   |      3 |    3 | Java |
|  6 | fff   |      3 |    3 | Java |
|  4 | ddd   |      4 |    4 | C#   |
|  5 | eee   |      4 |    4 | C#   |
|  8 | hhh   |   NULL | NULL | NULL |
|  9 | iii   |   NULL | NULL | NULL |
| 10 | jjj   |   NULL | NULL | NULL |
+----+-------+--------+------+------+

复制代码

右连接:RIGHT JOIN

同理右连接RIGHT JOIN就是求两个表的交集外加右表剩下的数据。再次从笛卡尔积的角度描述,右连接就是从笛卡尔积中挑出ON子句条件成立的记录,然后加上右表中剩余的记录(见最后一条)。

复制代码
SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;

+------+-------+--------+----+------------+
| id   | title | typeId | id | name       |
+------+-------+--------+----+------------+
|    1 | aaa   |      1 |  1 | C++        |
|    2 | bbb   |      2 |  2 | C          |
|    3 | ccc   |      3 |  3 | Java       |
|    4 | ddd   |      4 |  4 | C#         |
|    5 | eee   |      4 |  4 | C#         |
|    6 | fff   |      3 |  3 | Java       |
|    7 | ggg   |      2 |  2 | C          |
| NULL | NULL  |   NULL |  5 | Javascript |
+------+-------+--------+----+------------+

复制代码
外连接:OUTER JOIN

外连接就是求两个集合的并集。从笛卡尔积的角度讲就是从笛卡尔积中挑出ON子句条件成立的记录,然后加上左表中剩余的记录,最后加上右表中剩余的记录。另外MySQL不支持OUTER JOIN,但是我们可以对左连接和右连接的结果做UNION操作来实现。

复制代码
SELECT FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
UNION
SELECT
FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;

+------+-------+--------+------+------------+
| id   | title | typeId | id   | name       |
+------+-------+--------+------+------------+
|    1 | aaa   |      1 |    1 | C++        |
|    2 | bbb   |      2 |    2 | C          |
|    7 | ggg   |      2 |    2 | C          |
|    3 | ccc   |      3 |    3 | Java       |
|    6 | fff   |      3 |    3 | Java       |
|    4 | ddd   |      4 |    4 | C#         |
|    5 | eee   |      4 |    4 | C#         |
|    8 | hhh   |   NULL | NULL | NULL       |
|    9 | iii   |   NULL | NULL | NULL       |
|   10 | jjj   |   NULL | NULL | NULL       |
| NULL | NULL  |   NULL |    5 | Javascript |
+------+-------+--------+------+------------+

复制代码
USING子句

MySQL中连接SQL语句中,ON子句的语法格式为:table1.column_name = table2.column_name。当模式设计对联接表的列采用了相同的命名样式时,就可以使用 USING 语法来简化 ON 语法,格式为:USING(column_name)。
所以,USING的功能相当于ON,区别在于USING指定一个属性名用于连接两个表,而ON指定一个条件。另外,SELECT *时,USING会去除USING指定的列,而ON不会。实例如下。

复制代码
SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId =t_type.id;
+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
+----+-------+--------+----+------+

SELECT * FROM t_blog INNER JOIN t_type USING(typeId);
ERROR 1054 (42S22): Unknown column 'typeId' in 'from clause'
SELECT * FROM t_blog INNER JOIN t_type USING(id); -- 应为t_blog的typeId与t_type的id不同名,无法用Using,这里用id代替下。
+----+-------+--------+------------+
| id | title | typeId | name       |
+----+-------+--------+------------+
|  1 | aaa   |      1 | C++        |
|  2 | bbb   |      2 | C          |
|  3 | ccc   |      3 | Java       |
|  4 | ddd   |      4 | C#         |
|  5 | eee   |      4 | Javascript |
+----+-------+--------+------------+

复制代码
自然连接:NATURE JOIN

自然连接就是USING子句的简化版,它找出两个表中相同的列作为连接条件进行连接。有左自然连接,右自然连接和普通自然连接之分。在t_blog和t_type示例中,两个表相同的列是id,所以会拿id作为连接条件。
另外千万分清下面三条语句的区别 。
自然连接:SELECT FROM t_blog NATURAL JOIN t_type;
笛卡尔积:SELECT
FROM t_blog NATURA JOIN t_type;
笛卡尔积:SELECT * FROM t_blog NATURE JOIN t_type;

复制代码
SELECT * FROM t_blog NATURAL JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog,t_type WHERE t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type USING(id);

+----+-------+--------+------------+
| id | title | typeId | name       |
+----+-------+--------+------------+
|  1 | aaa   |      1 | C++        |
|  2 | bbb   |      2 | C          |
|  3 | ccc   |      3 | Java       |
|  4 | ddd   |      4 | C#         |
|  5 | eee   |      4 | Javascript |
+----+-------+--------+------------+

SELECT * FROM t_blog NATURAL LEFT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type USING(id);

+----+-------+--------+------------+
| id | title | typeId | name       |
+----+-------+--------+------------+
|  1 | aaa   |      1 | C++        |
|  2 | bbb   |      2 | C          |
|  3 | ccc   |      3 | Java       |
|  4 | ddd   |      4 | C#         |
|  5 | eee   |      4 | Javascript |
|  6 | fff   |      3 | NULL       |
|  7 | ggg   |      2 | NULL       |
|  8 | hhh   |   NULL | NULL       |
|  9 | iii   |   NULL | NULL       |
| 10 | jjj   |   NULL | NULL       |
+----+-------+--------+------------+

SELECT * FROM t_blog NATURAL RIGHT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type USING(id);

+----+------------+-------+--------+
| id | name       | title | typeId |
+----+------------+-------+--------+
|  1 | C++        | aaa   |      1 |
|  2 | C          | bbb   |      2 |
|  3 | Java       | ccc   |      3 |
|  4 | C#         | ddd   |      4 |
|  5 | Javascript | eee   |      4 |
+----+------------+-------+--------+

复制代码
补充

博客开头给出的第一张图除去讲了的内连接、左连接、右连接、外连接,还有一些特殊的韦恩图,这里补充一下。

复制代码
SELECT FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_type.id IS NULL;
+----+-------+--------+------+------+
| id | title | typeId | id | name |
+----+-------+--------+------+------+
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
+----+-------+--------+------+------+
SELECT
FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_blog.id IS NULL;
+------+-------+--------+----+------------+
| id | title | typeId | id | name |
+------+-------+--------+----+------------+
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+----+------------+
SELECT FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_type.id IS NULL
UNION
SELECT
FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_blog.id IS NULL;
+------+-------+--------+------+------------+
| id | title | typeId | id | name |
+------+-------+--------+------+------------+
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+------+------------+