1、通过WHERE进行简单连接
SELECT * FROM company3,department 不添加WHERE将会显示所有数据
SELECT * FROM company3,department WHERE company3.no = department.id; 通过WHERE构建匹配逻辑
SELECT company3.name,company3.salary,department.dept
FROM company3,department WHERE company3.no = department.id; 显示指定字段
通过select+where，创建多个表的连接
2、内连接(INNER JOIN)
CREATE TABLE employees(
id int PRIMARY KEY,
name text,
age int CHECK(age > 0),
address text,
salary numeric CHECK(salary > 0)
);
INSERT INTO employees VALUES(1,‘王大‘,25,‘beijing‘,10000),(2,‘张三‘,
(3,‘李四‘,21,15000),(4,‘李二‘,28,‘shenzhen‘,
(5,‘王五‘,24,‘shanghai‘,20000),(6,‘杨三‘,19,
(7,‘张四‘,22,(8,‘杨四‘,20,10000);
创建表格1
CREATE TABLE department(id int,dept text,fac_id int);
INSERT INTO department VALUES(1,‘IT‘,1);
INSERT INTO department VALUES(2,‘Engineering‘,2);
INSERT INTO department VALUES(3,‘HR‘,7);
INSERT INTO department VALUES(10,‘Market‘,10);
创建表格2
SELECT employees.id,employees.name,department.dept
FROM employees INNER JOIN department
ON employees.id = department.id;
基于两个表格的id字段，连接表格，取交集
3、全连接(FULL OUTER JOIN)
SELECT employees.id,department.dept
FROM employees FULL OUTER JOIN department
ON employees.id = department.id;
基于两个表格的id字段，连接表格，取并集，缺失值为NULL
4、左外连接(LEFT OUTER JOIN)
SELECT employees.id,department.dept
FROM employees LEFT OUTER JOIN department
ON employees.id = department.id;
连接表格后，保留employees的所有数据条目
5、右外连接(RIGHT OUTER JOIN)
SELECT employees.id,department.dept
FROM employees RIGHT OUTER JOIN department
ON employees.id = department.id;
连接表格后，保留department的所有数据条目
6、交叉连接
SELECT employees.id,dept,salary FROM employees CROSS JOIN department;
笛卡尔积：检索出的行的数目为第一个表中的行数乘以第二个表中的行数
当两个表有共同名称的字段（比如id），将会报错，所以这里需要指示，例如employees.id
多个连接条件
SELECT employees.id,department.dept
FROM employees INNER JOIN department
ON employees.id = department.id
AND employees.id = department.fac_id;
JOIN连接三个表
CREATE TABLE education(name text,edu text);
INSERT INTO education VALUES(‘王大‘,‘本科‘),(‘张三‘,
(‘李四‘,‘硕士‘),(‘李二‘,
(‘王五‘,‘PHD‘),(‘杨三‘,
(‘张四‘,(‘杨四‘,‘本科‘);
创建表格3
SELECT employees.id,employees.age,employees.salary,department.dept
FROM (employees INNER JOIN education ON employees.name = education.name)
FULL OUTER JOIN department ON employees.id = department.id;
注意嵌套写法
