sql查詢 ——聚合函數

--聚合函數
      -- sum()
      -- 求和
      select sum(age) from student;

      -- count()
      -- 求數量
      -- 數據量
      select count(*) as '數量' from student;

      -- max()
      --最大值
      select max(age) as '最大值' from student;

      -- min
      -- 最小值
      select min(age) as '最小值' from student;

      -- avg()
      --求平均值
      select avg(age) as '平均值' from student;

      --round()
      --保留幾位小數
      select round(avg(age),2) as '平均值2位小數' from student;

      -- 綜合查詢
      select sum(age)/count(age) as '平均值' from student;