MySQL的JOIN

Posted 2021-06-20 willem_chen

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mysql的JOIN

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。

在这里插入图片描述

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 |
    +------+-------+--------+------+------------+

创建表

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 |
    +----+------------+

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。

1、SELECT * FROM t_blog CROSS JOIN t_type;
2、SELECT * FROM t_blog INNER JOIN t_type;
3、SELECT * FROM t_blog,t_type;
4、SELECT * FROM t_blog NATURE JOIN t_type;
5、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 |
    +----+-------+--------+----+------------+

①交叉连接查询笛卡尔积

SELECT * FROM product,category;

查出来的是两张表的乘积（查出来的结果没有意义）

过滤出有意义的
SELECT * FROM product,category WHERE cno=cid;
或者
SELECT * FROM product AS p,category AS c WHERE p.cno=c.cid;

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。

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#   |
    +----+-------+--------+----+------+

①内连接查询

隐式内链接:

SELECT * FROM product AS p,category AS c WHERE p.cno=c.cid;

显示内连接:

SELECT * FROM product AS p INNER JOIN category AS c ON p.cno=c.cid;

区别：

隐式内连接：在查询出结果的基础上去做的WHERE条件过滤
显示内连接：带着条件去查询结果，查询效率要高

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）。

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 |
    +----+-------+--------+------+------+

①左外连接

将左表中的所有数据都查询出来，如果右表中没有对应的数据，用NULL代替。

SELECT * FROM product p LEFT OUTER JOIN category c ON p.cno= c.cid;

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

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