27.MySQL中的自连接

1.简介

自连接查询是内连接中的一种特殊查询。

它是指相互连接的表在物理上为同一个表，但逻辑上分为两个表。

2.准备

CREATE DATABASE mahaiwuji;
USE mahaiwuji;

CREATE TABLE student(
    id INT(4) PRIMARY KEY,
    name VARCHAR(36),
    gid INT(4)
) ENGINE = INNODB DEFAULT CHARSET = utf8;

INSERT INTO student VALUES (1,'a1',1);
INSERT INTO student VALUES (2,'a2',1);
INSERT INTO student VALUES (3,'a3',1);
INSERT INTO student VALUES (4,'a4',1);
INSERT INTO student VALUES (5,'a5',2);
INSERT INTO student VALUES (6,'a6',2);
INSERT INTO student VALUES (7,'a7',2);
INSERT INTO student VALUES (8,'a8',2);

s27_1.png

3.案例

查找和a1同一个gid的数据

SELECT * FROM student s1
JOIN student s2
ON s2.`name`='a1' AND s1.gid = s2.gid;

s27_2.png

去掉不要的列

SELECT s1.id,s1.name FROM student s1
JOIN student s2
ON s2.`name`='a1' AND s1.gid = s2.gid

s27_3.png