SQL语句的多表查询方式

例如：按照department_id查询employees(员工表)和departments(部门表) 的信息。

方式一(通用型):SELECT ... FROM ... WHERE

SELECT st_name,e.department_id,d.department_name

FROM employees e,departments d

where e.department_id = d.department_id

方式二：SELECT ... FROM ... NATURAL JOIN ...

有局限性：会自动连接两个表中相同的列(可能有多个:department_id和manager_id)

SELECT last_name,department_id,department_name

FROM employees

NATURAL JOIN departments

方式三：SELECT ... JOIN ... USING ...

有局限性：好于方式二，但若多表的连接列列名不同，此法不合适 SELECT last_name,department_id,department_name

FROM employees

JOIN departments

USING(department_id)

方式四：SELECT ... FROM ... JOIN ... ON ...

常用方式，较方式一，更易实现外联接(左、右、满)

SELECT last_name,e.department_id,department_name

FROM employees e

JOIN departments d

ON e.department_id = d.department_id

--内连接

1）

--等值连接

--不等值连接

2）

--非自连接

--自连接

--外连接

--左外连接、右外连接、满外连接