w3resource_MySQL练习：Joins

w3resource_MySQL练习题：Joins

1. Write a query to find the addresses (location_id, street_address, city, state_province, country_name) of all the departments

Hint : Use NATURAL JOIN.

Sample table: locations

Sample table: countries

-- 1. 自然连接，MySQL自己判断多表的连接条件，分为内/外自然连接
select location_id, street_address, city, state_province, country_name
from locations
NATURAL JOIN countries
-- 2. 多表连接，在from里连接多张表，通过where进行匹配
select l.location_id, l.street_address, l.city, l.state_province, c.country_name
from locations as l, countries as c
where l.country_id=c.country_id

2. Write a query to find the name (first_name, last name), department ID and name of all the employees

Sample table: employees

Sample table: departments

-- 1. 多表连接
select e.first_name, e.last_name, e.department_id, d.department_name
from employees as e, departments as d
where e.department_id=d.department_id
-- 2. 左连接（显然employees表数量远大于departments表）
select e.first_name, e.last_name, e.department_id, d.department_name
from employees as e
left join departments as d
on e.department_id=d.department_id

　

3. Write a query to find the name (first_name, last_name), job, department ID and name of the employees who works in London

Sample table: departments

Sample table: locations

-- 1. 多表连接
select e.first_name, e.last_name, e.job_id, e.department_id, d.department_name
from employees as e, departments as d, locations as l
where e.department_id=d.department_id
and d.location_id=l.location_id
and l.city='London'
-- 2. 左连接
select e.first_name, e.last_name, e.job_id, e.department_id, d.department_name
from employees as e
left join departments as d
on e.department_id=d.department_id
left join locations as l
on d.location_id=l.location_id
where l.city='London'

4. Write a query to find the employee id, name (last_name) along with their manager_id and name (last_name)

Sample table: employees

-- 1. 使用多表连接
select e1.employee_id, e1.last_name, e2.employee_id, e2.last_name
from employees as e1, employees as e2
where e1.manager_id=e2.employee_id
-- 2. 使用内连接
SELECT e.employee_id 'Emp_Id', e.last_name 'Employee',
m.employee_id 'Mgr_Id', m.last_name 'Manager'
FROM employees e
join employees m
ON (e.manager_id = m.employee_id);

　　

5. Write a query to find the name (first_name, last_name) and hire date of the employees who was hired after 'Jones'

Sample table: employees

-- 1. 通过where进行筛选
select first_name, last_name, hire_date
from employees
where hire_date>(
    select hire_date from employees where last_name='Jones'
)
-- 2. 内连接
SELECT e.first_name, e.last_name, e.hire_date
FROM employees e
JOIN employees davies
ON (davies.last_name = 'Jones')
WHERE davies.hire_date < e.hire_date;

6. Write a query to get the department name and number of employees in the department

Sample table: employees

Sample table: departments

-- 要点：多表连接后，在新表做group by
select d.department_name, count(*)
from employees as e, departments as d
where e.department_id=d.department_id
group by e.department_id

　　

7. Write a query to find the employee ID, job title, number of days between ending date and starting date for all jobs in department 90

Sample table: employees

-- 要点：datediff()
SELECT employee_id, j.job_title, DATEDIFF(end_date, start_date)
FROM job_history
NATURAL JOIN jobs AS j
WHERE department_id = 90;

　　

8. Write a query to display the department ID and name and first name of manager

Sample table: employees

Sample table: departments

-- 要点：以departments表为左表
select d.department_id, d.department_name, e.first_name
from departments as d
left join employees as e
won d.manager_id=e.employee_id

　　

9. Write a query to display the department name, manager name, and city

Sample table: employees

Sample table: departments

Sample table: locations

-- 要点：以departments表为主表
select d.department_name, e.first_name, l.city
from departments as d
left join employees as e
on d.manager_id=e.employee_id
left join locations as l
on d.location_id=l.location_id

10. Write a query to display the job title and average salary of employees

Sample table: employees

-- 要点：group by分组计算组内均值
select job_id, avg(salary)
from employees
group by job_id

11. Write a query to display job title, employee name, and the difference between salary of the employee and minimum salary for the job

Sample table: employees

-- 要点：多表连接
SELECT job_title, first_name, salary-min_salary 'Salary - Min_Salary'
FROM employees
NATURAL JOIN jobs;

12. Write a query to display the job history that were done by any employee who is currently drawing more than 10000 of salary

Sample table: employees

Sample table: Job_history

-- 要点：多表连接
select j.*
from job_history as j, employees as e
where j.employee_id=e.employee_id
and e.salary>10000

　　

13. Write a query to display department name, name (first_name, last_name), hire date, salary of the manager for all managers whose experience is more than 15 years

Sample table: employees

Sample table: departments

-- 要点：datediff
select e.first_name, e.last_name, e.hire_date, e.salary, datediff(now(), (e.hire_date)) as exp
from departments as d, employees as e
where d.manager_id=e.employee_id
and datediff(now(), (e.hire_date))>15