多表查询:

1. 笛卡尔积的形式;

2. 内连接的形式:

查询两个表中符合连接条件的记录;

SELECT 字段名称,... FROM tbl_name1

INNER JOIN tbl_name2

ON 连接条件

3. 外连接的形式:

1).左外连接:

SELECT 字段名称,... FROM tbl_name1

LEFT [OUTER] JOIN tbl_name2

ON 条件;

//先显示左表中的全部记录，再去右表中查询复合条件的记录，不符合的以NULL代替

2).右外连接:

SELECT 字段名称,... FROM tbl_name1

RIGHT [OUTER] JOIN tbl_name2

ON 条件;

//先显示右表中的全部记录，再去左表中查询复合条件的记录，不符合的以NULL代替

案例：

CREATE DATABASE IF NOT EXISTS test2 DEFAULT CHARACTER SET 'UTF8';

USE test2;

CREATE TABLE emp(

id INT UNSIGNED AUTO_INCREMENT KEY,

username VARCHAR(20) NOT NULL UNIQUE COMMENT '编号',

age TINYINT UNSIGNED NOT NULL DEFAULT 18 COMMENT '年龄',

sex ENUM('男','女','保密') NOT NULL DEFAULT '保密' COMMENT '性别',

addr VARCHAR(20) NOT NULL DEFAULT '北京',

depId TINYINT UNSIGNED NOT NULL COMMENT '部门对应的编号'

)ENGINE=INNODB CHARSET=UTF8;

INSERT emp(username,age,depId) VALUES('king',24,1),

('queen',25,2),

('haha',26,1),

('lily',27,1),

('rose',28,3),

('john',29,3);

INSERT emp(username,age,depId) VALUES('测试用户',39,6);

CREATE TABLE dep(

id TINYINT UNSIGNED AUTO_INCREMENT KEY,

depName VARCHAR(50) NOT NULL UNIQUE,

depDesc VARCHAR(100) NOT NULL DEFAULT ''

)ENGINE=INNODB CHARSET=UTF8;

INSERT dep(depName,depDesc) VALUES('研发部','技术攻关'),

('JAVA部','撸JAVA代码'),

('WEB前端部','撸前端代码'),

('IOS部','撸IOS代码');

//查询 emp id username age 部门名称 dep depName

//笛卡尔积的形式:

SELECT emp.id,emp.username,emp.age,dep.depName FROM emp,dep;

//内连接的形式

SELECT e.id,e.username,e.age,d.depName

FROM emp AS e

INNER JOIN dep AS d

ON e.depId=d.id;

//查询emp id username age addr dep id depName depDesc

SELECT e.id,e.username,e.age,e.addr,

d.id,d.depName,d.depDesc

FROM dep AS d

JOIN emp AS e

ON d.id=e.depId;

//测试左外连接

SELECT e.id,e.username,e.age,d.depName,d.depDesc

FROM emp AS e

LEFT OUTER JOIN dep AS d

ON e.depId=d.id;

// 测试右外连接

SELECT e.id,e.username,e.age,d.depName,d.depDesc

FROM emp AS e

RIGHT JOIN dep AS d

ON e.depId=d.id;