分组统计

分组统计最常跟聚合函数COUNT、SUM、AVG、MIN、MAX一起使用。

• SUM、AVG、MIN、MAX

  select emp_no
    ,SUM(salary) AS salary_sum
  ,AVG(salary) AS salary_avg
  ,MIN(salary) AS salary_min
  ,MAX(salary) AS salary_max
  from salaries
  where from_date > '2002-01-01'
  group by emp_no
  limit 5;

  

• COUNT可以用来计数，COUNT(DISTINCT ···）可以进行去重计数

如我们可以查询salaries中一共有多少行数据，一共有多少个员工。

select count(*) as rc
    ,count(distinct emp_no) as emp_num
from salaries;