Mysql Join 总结

Mysql 的连接 分为 ：  内连接 、外连接 、左连接、右连接、自连接

 

数据准备：

 1.创建博客表

 CREATE TABLE t_blog(
        id INT PRIMARY KEY AUTO_INCREMENT,
        title VARCHAR(50),
        typeId INT
    );


insert into  t_blog(title,typeId) values("aaa",1);
insert into  t_blog(title,typeId) values("bbb",2);
insert into  t_blog(title,typeId) values("ccc",3);
insert into  t_blog(title,typeId) values("ddd",4);
insert into  t_blog(title,typeId) values("eee",4);
insert into  t_blog(title,typeId) values("fff",3);
insert into  t_blog(title,typeId) values("ggg",2);
insert into  t_blog(title,typeId) values("hhh",NULL);
insert into  t_blog(title,typeId) values("iii",NULL);
insert into  t_blog(title,typeId) values("jjj",NULL);

2.创建博客类型表

  CREATE TABLE t_type(
        id INT PRIMARY KEY AUTO_INCREMENT,
        name VARCHAR(20)
    );

insert into t_type(name) value("C++");
insert into t_type(name) value("C");
insert into t_type(name) value("Java");
insert into t_type(name) value("C#");
insert into t_type(name) value("Javascript");

a. 笛卡尔乘积

  左边10条数据M，右边5条数据N，总共50条数据: M*N=50

  SELECT * FROM t_blog CROSS JOIN t_type;

  SELECT * FROM t_blog INNER JOIN t_type;

  SELECT * FROM t_blog,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 |
    +----+-------+--------+----+------------+

b.内连接

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

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

c.左连接

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

d.外连接

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。

UNIO ： 连接多个select 查询结果集，并去重

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

e.自连接

 自己连接自己

f.其他特殊情况

   1.左连接 只取左边部分

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

  2. 左右外连接 只取左右

 

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