一、笛卡尔积的多表查询

1.案例 查询员工名为’Abel’的员工在哪个城市工作?

 2.出现笛卡尔积（交叉连接）的错误

select employee_id, department_name
from employees,departments

为什么出现2889条记录？

 

Employees 107    Departments 27

每个员工和每个部分都匹配了一遍 107*27=2889

缺少了多表连接的条件

3.多表查询的正确方式，需要有连接条件

①两个表的连接条件

select last_name,department_name
from employees e,departments d
where e.department_id = d.department_id

②两个表的字段名相同(where 条件后面的)，需要指明字段的表

select last_name,department_name,e.department_id
from employees e,departments d
where e.department_id = d.department_id

③sql优化:建议多表查询时，每个字段指明其所在的表

select e.last_name,e.department_id,d.department_name
from employees e,departments d
where e.department_id = d.department_id

④可以给表起别名，可以在select和where使用表的别名。起了别名不能使用原名。

⑤n个表实现多表查询，至少n-1个连接条件

4.查询员工的department_id,last_name,department_name,city

select e.employee_id,e.last_name,d.department_name,l.city
from employees e,departments d,locations l
where e.department_id=d.department_id and d.location_id=l.location_id

二、等值连接和非等值连接，自连接和非自连接

1.多表查询的分类

角度1：等值连接 VS 非等值连接

角度2：自连接 VS 非自连接

角度3：内连接 VS 外连接

2.等值连接 VS 非等值连接

①非等值连接的举例：查询员工的工资等级

select last_name,salary,grade_level
from employees e,job_grades j
where salary between j.lowest_sal and j.highest_sal
或
select last_name,salary,grade_level
from employees e,job_grades j
where salary >= lowest_sal and salary<=highest_sal

3. 自连接 VS 非自连接

自连接：1个表。表自己连接自己

非自连接：多个表

①查询员工id,员工姓名和主管姓名

select e1.employee_id,e1.last_name,e2.last_name manager_name
from employees e1,employees e2
where e1.manager_id=e2.employee_id

 4.内连接 VS 外连接

左表：员工表employees，有些员工没有部门

右表：部门表departments，有些部门存在，没有左表的员工

①内连接：以上练习都是内连接。交集

107个人，查询106个记录。左表和右表满足查询条件的结果

select employee_id,department_name
from employees e,departments d
where e.department_id=d.department_id

②外连接：并集

左外连接：查询所有的员工的last_name,epartment_name信息（所有的就是外连接）

select employee_id,department_name
from employees e
left join departments d on e.department_id=d.department_id

 ③右外连接：

select employee_id,department_name
from employees e
right join departments d on e.department_id=d.department_id

 三、7种SQL JOINS的实现

左查询：左表

右查询：右表

 【练习题】

 

1.
select e.last_name,e.department_id,d.department_name
from employees e
left join departments d on e.department_id = d.department_id


2.
select e.employee_id,e.job_id,d.location_id
from employees e
join departments d on d.department_id = e.department_id
where e.department_id = 90


3.
select e.last_name,e.commission_pct,d.department_name,d.location_id,l.city
from employees e
join departments d on e.department_id=d.department_id
join locations l on l.location_id=d.location_id
where e.commission_pct is not null

4.
select e.last_name,e.job_id,e.department_id,d.department_name,l.city
from employees e
join departments d on e.department_id=d.department_id
join locations l on l.location_id=d.location_id
where l.city = 'Toronto'

5.
select d.department_name,l.street_address,e.last_name,e.job_id,e.salary
from employees e
join departments d on e.department_id=d.department_id
join locations l on l.location_id=d.location_id
where d.department_name='Executive'

6.
select e1.last_name "employees",e1.employee_id "Emp#",e2.last_name "manager",e2.employee_id "#Mgr"
from employees e1,employees e2
where e1.manager_id = e2.employee_id and e1.last_name='Kochhar'
或
select e1.last_name "employees",e1.employee_id "Emp#",e2.last_name "manager",e2.employee_id "#Mgr"
from employees e1
join employees e2 on e1.manager_id = e2.employee_id
where e1.last_name='Kochhar'

7.
select e.last_name,d.department_name
from employees e
right join departments d on e.department_id = d.department_id
where e.last_name is null


8.
select d.department_name,l.city
from departments d
right join locations l on d.location_id = l.location_id
where department_name is null


9.
select e.employee_id,e.last_name,e.salary,d.department_name
from employees e
join departments d on d.department_id = e.department_id
where d.department_name = 'IT' or  d.department_name = 'Sales'
或
select e.employee_id,e.last_name,e.salary,d.department_name
from employees e
join departments d on d.department_id = e.department_id
where d.department_name in ('Sales','IT')