PostgreSQL-7-数据连接

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',25,'beijing',10000),

       (3,'李四',21,'beijing',15000),(4,'李二',28,'shenzhen',10000),

       (5,'王五',24,'shanghai',20000),(6,'杨三',19,'shanghai',15000),

       (7,'张四',22,'shenzhen',20000),(8,'杨四',20,'beijing',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,employees.name,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,employees.name,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,employees.name,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,employees.name,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','本科'),

       ('李四','硕士'),('李二','本科'),

       ('王五','PHD'),('杨三','硕士'),

       ('张四','PHD'),('杨四','本科');

创建表格3

SELECT employees.id,employees.name,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