MySql basic query-grouping function

#Grouping function
#1. Query the maximum, minimum, average, and total wages of the company's employees #Salary
SELECT SUM(salary),MAX(salary),MIN(salary),AVG(salary),COUNT(salary) FROM employees
deduplication

SELECT
    sum( DISTINCT salary ),
    sum( salary ) 
FROM
    employees

#Query have several types of wages
SELECT count(DISTINCT salary) FROM employees
#Statistics of the number of wages #The
SELECT count(salary) FROM employees
total number of rows is the total number of records

SELECT count(*) FROM employees
SELECT count(1) FROM employees

#2. Query the difference between the maximum entry time and the minimum entry time in the employee table (DIFFRENCE)

SELECT
    datediff(
        max( hiredate ),
    min( hiredate )) AS '相差天数' 
FROM
    employees

#3. Query the number of employees whose department number is 90
SELECT count(*) FROM employees WHERE department_id=90