Oracle——多表查询

一、内连接和外连接

• 内连接: 合并具有同一列的多个表的行, 结果集中不包含一个表与另一个表不匹配的行
• 外连接: 两个表在连接过程中除了返回满足连接条件的行以外还返回左（或右）表中不满足条件的行 ，这种连接称为左（或右） 外连接。没有匹配的行时, 结果表中相应的列为空(NULL). 外连接的 WHERE 子句条件类似于内部连接, 但连接条件中没有匹配行的表的列后面要加外连接运算符, 即用圆括号括起来的加号(+).

二、内连接

等值连接

select employee_id,e.department_id,department_name
from employees e join departments d 
--using(department_id)--两个表连接列的名必须必须相同
on e.department_id = d.department_id

三表连接

select employee_id,e.department_id,department_name,city
from employees e join departments d 
on e.department_id = d.department_id
join locations l
on d.location_id = l.location_id

非等值连接

--多表查询员工薪资的档次
select last_name ,salary ,j.grade_level
from employees,job_grades j
where salary between j.lowest_sal and j.highest_sal

三、外连接

左（右）外连接的两种方式

--左外连接
select employee_id,d.department_id,d.department_name
from employees e ,departments d
where e.department_id = d.department_id(+)

--左外连接
select employee_id,d.department_id,d.department_name
from employees e left join departments d
on e.department_id = d.department_id

满外连接

两个表在连接过程中除了返回满足连接条件的行以外还返回两个表中不满足条件的行 ，这种连接称为满外连接

--满外连接
select employee_id,e.department_id,department_name
from employees e full join departments d 
on e.department_id = d.department_id

四、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 e.last_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