条件聚合查询

Question

我见过其他解决方案。好奇这种解决这些问题的方法是否有效。

表格：

attendance_events : 日期 | student_id |出席人数

all_students : student_id | school_id | grade_level | date_of_birth |家乡

百分之几的学生在生日那天上学？

With agg_join as (SELECT att.date as dates, att.attendance as attendance, als.date_of_birth as DOB, att.student_id as student_id
      FROM attendance_events att
      join all_students als on att.student_id = als.studentid)
      
Select count(DISTINCT student_id) as total_students, 
  count( Distinct case when DOB = dates and attendance = TRUE) as count_of_DOBS,
  total_students/ count_of_DOBS as percent_of_student
from agg_join

从昨天到今天，哪个年级的出勤率下降幅度最大？

With agg_join as ( SELECT att.date as dates, att.attendance as attendance, als.grade_level as grade
      FROM attendance_events att
      join all_students als on att.student_id = als.studentid)
      
Select grade,  
      case when dates ( 'd', -1, currentdate) and attendance = True then 1
      else 0 end as yesterday_att,

       case when dates ( 'd', currentdate) and attendance = True then 1
      else 0 end as Today_att, 
      
      (Today_att - yesterday_att) * -1 AS DIFF
      
from agg_join
Group by grade
Order by DIFF DESC
Limit 1

Answer 1

百分之几的学生在生日那天上学？

SELECT 100.0
     * count(*) FILTER (WHERE EXISTS (SELECT FROM attendance_events a
                                      WHERE a.student_id = s.student_id
                                      AND f_mmdd(a.date) = f_mmdd(s.date_of_birth)
                                     ))
     / count(*) AS percentage
FROM   all_students s;

这里定义自定义函数f_mmdd()的地方：

How do you do date math that ignores the year?

参见：

Convert numeric result to percentage with decimal digits

关于聚合FILTER条款：

Aggregate columns with additional (distinct) filters

从昨天到今天，哪个年级的出勤率下降幅度最大？

SELECT s.grade_level
     , count(*) FILTER (WHERE a.date = CURRENT_DATE - 1) AS yesterday_attendance
     , count(*) FILTER (WHERE a.date = CURRENT_DATE) AS today_attendance
FROM   attendance_events a
JOIN   all_students s USING (student_id)
WHERE  a.date IN (CURRENT_DATE, CURRENT_DATE -1)  -- logically redundant
GROUP  BY s.grade_level
ORDER  BY today_attendance - yesterday_attendance
    -- short for: yesterday_attendance - today_attendance DESC
LIMIT  1;

WHERE a.date IN (CURRENT_DATE, CURRENT_DATE -1)在逻辑上是多余的，但查询速度更快。

阅读并尝试理解为什么这些工作，如果你不是自己解决的话。

条件聚合查询

Conditional aggregate queries

sql

postgresql

join

aggregate-filter

百分之几的学生在生日那天上学？

从昨天到今天，哪个年级的出勤率下降幅度最大？

百分之几的学生在生日那天上学？

从昨天到今天，哪个年级的出勤率下降幅度最大？