Mysql--七种 Join 查询

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 1 准备数据

1.1 建立 Employee 表

DROP TABLE IF EXISTS employee;
CREATE TABLE IF NOT EXISTS employee(
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(40),
    dept_id INT
);
INSERT INTO employee(name, dept_id) VALUES('Alice', 1);
INSERT INTO employee(name, dept_id) VALUES('BOb', 2);
INSERT INTO employee(name, dept_id) VALUES('Chris', 3333);
INSERT INTO employee(name, dept_id) VALUES('David', 4444);

1.2 建立 Department 表

DROP TABLE IF EXISTS department;
CREATE TABLE IF NOT EXISTS department(
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(40)
);
INSERT INTO department(name) VALUES('RD');
INSERT INTO department(name) VALUES('HR');
INSERT INTO department(name) VALUES('test01');
INSERT INTO department(name) VALUES('test02');

2 再回顾一下数据

2.1 Employee表

SELECT * FROM employee;

2.2 Department表

SELECT * FROM department;

3 测试

3.1 AB共有

图示说明:

Sql查询:

SELECT * FROM employee e INNER JOIN department d WHERE e.dept_id = d.id;

结果:

3.2 左外连接

图示说明:

 

Sql查询:

SELECT * FROM employee e LEFT JOIN department d ON e.dept_id = d.id;

结果:

左外连接:左边一定有,如果右边没有则为Null

3.3 右外连接

图示说明:

Sql查询:

SELECT * FROM employee e RIGHT JOIN department d ON e.dept_id = d.id;

结果:

右外连接:右边一定有,如果左边没有则为Null

3.4 A独有

图示说明:

Sql查询:

SELECT * FROM employee e LEFT JOIN department d ON e.dept_id = d.id WHERE d.id IS NULL;

结果:

左边独有,说明右边为空,于是对右边加上 IS NULL 的条件即可

3.5 B独有

图示说明:

Sql查询:

SELECT * FROM employee e RIGHT JOIN department d ON e.dept_id = d.id WHERE e.id IS NULL;

结果:

右边独有,说明左边为空,于是对左边加上 IS NULL 的条件即可

3.6 AB并集

图示说明:

Sql查询:

SELECT * FROM employee e LEFT JOIN department d ON e.dept_id = d.id
UNION
SELECT * FROM employee e RIGHT JOIN department d ON e.dept_id = d.id;

结果:

其实就是 3.2(左外连接) 与 3.3(右外连接) 两者的并集

3.7 AB分别独有的并集

图示说明:

Sql查询:

SELECT * FROM employee e LEFT JOIN department d ON e.dept_id = d.id WHERE d.id IS NULL
UNION
SELECT * FROM employee e RIGHT JOIN department d ON e.dept_id = d.id WHERE e.id IS NULL;

结果:

其实就是 3.4(A独有) 与 3.5(B独有) 两者的并集

======下一篇======

Mysql--存储引擎(MyISam & InnoDB)

posted @ 2018-04-24 11:52  cyhbyw  阅读(322)  评论(0编辑  收藏  举报