w3resource_MySQL练习: Aggregate_functions

w3resource_MySQL练习题：Aggregate_functions

 

1. Write a query to list the number of jobs available in the employees table

Sample table: employees

-- 要点：count() + distinct
select count(distinct job_id)
from employees

2. Write a query to get the total salaries payable to employees

Sample table: employees

-- 要点：sum()
select sum(salary)
from employees

 

3. Write a query to get the minimum salary from employees table

Sample table: employees

-- 要点：min()
select min(salary)
from employees

4. Write a query to get the maximum salary of an employee working as a Programmer

Sample table: employees

-- 要点：max()
select max(salary)
from employees
where job_id = 'IT_PROG'

　　

5. Write a query to get the average salary and number of employees working the department 90

Sample table: employees

-- 要点：avg() + count()
select avg(salary), count(*)
from employees
where department_id=90

6. Write a query to get the highest, lowest, sum, and average salary of all employees

Sample table: employees

-- 要点：max() + min() + sum() + avg()
select max(salary), min(salary), sum(salary), avg(salary)

　　

7. Write a query to get the number of employees with the same job

Sample table: employees

-- 要点：group by进行分组
select job_id, count(*)
from employees
group by job_id

8. Write a query to get the difference between the highest and lowest salaries

Sample table: employees

-- 要点：max() + min()
select max(salary)-min(salary)
from employees

　　

9. Write a query to find the manager ID and the salary of the lowest-paid employee for that manager

Sample table: employees

-- 要点：根据manager_id进行分组，得到每个分组内最低的salary(min)
select manager_id, min(salary)
from employees
group by manager_id

　　

10. Write a query to get the department ID and the total salary payable in each department

Sample table: employees

-- 要点：根据department_id进行分组，得到每个分组内salary总和(sum)
select department_id, sum(salary)
from employees
group by department_id

　　

11. Write a query to get the average salary for each job ID excluding programmer

Sample table: employees

-- 要点：通过job_id进行分组，得到每个分组内平均的salary(avg)，
select job_id, avg(salary)
from employees
where job_id<>'IT_PROG'
group by job_id

　　

12. Write a query to get the total salary, maximum, minimum, average salary of employees (job ID wise), for department ID 90 only

Sample table: employees

-- 要点：同上，通过job_id进行分组，得到每个组内相应数值，并且使用where进行department_id筛选
select job_id, sum(salary), max(salary), min(salary), avg(salary)
from employees
where department_id=90
group by job_id

13. Write a query to get the job ID and maximum salary of the employees where maximum salary is greater than or equal to $4000

Sample table: employees

-- 要点：分组后的条件限制，使用having
select job_id, max(salary)
from employees
group by job_id
having max(salary)>=4000

　　

14. Write a query to get the average salary for all departments employing more than 10 employees

Sample table: employees

-- 要点：同上，分组后使用having
select department_id, avg(salary)
from employees
group by department_id
having count(salary)>10