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