分组函数

SELECT SUM(employees.salary) FROM employees; 

SELECT AVG(employees.salary) FROM employees;

SELECT MAX(employees.salary) FROM employees;

SELECT MIN(employees.salary) FROM employees;

SELECT COUNT(employees.salary) FROM employees;

SELECT SUM(employees.salary) 和,AVG(employees.salary) 平均,MAX(employees.salary) 最大值,MIN(employees.salary) 最小值,COUNT(employees.salary) 数目 FROM employees;

补充：

所有的分组函数都忽略NULL值

和去重结合

SELECT SUM(DISTINCT employees.salary) FROM employees;

SELECT COUNT(DISTINCT employees.salary),COUNT(employees.salary) FROM employees;