[打卡]聚合函数

2024-04-23 12:15:17

#1.avg/sum:只适用于数值类型的字段或变量
SELECT AVG(salary),SUM(salary)
from employees;
#字符串求和没有任何意义
SELECT AVG(last_name),SUM(last_name),AVG(hire_date),SUM(hire_date)
from employees;
#2.max/min
SELECT MAX(salary),MIN(salary)
from employees;

SELECT MAX(last_name),MIN(last_name),MAX(hire_date),MIN(hire_date)
from employees;
#3.作用：计算指定字段在查询结果中出现的个数
SELECT COUNT(employee_id)
from employees;