如何将来自不同表的这两个查询合并为一个来计算百分比？

Question

我有以下查询，其中有学生出勤率：

select total_presences from diary.period_attendance 
where id_customer = 1492 and id_diary_period = 172818 and id_user = 835603;

而且我有同期的课时数

select count(*) from diary.lesson where id_diary_period =  and id_customer =  and end_date < now();

我想用 total_presences 除以课时数得到学生的出勤率。

如何在单个查询中执行此操作？

Answer 1

您可以使用交叉连接或联合

 SELECT total_presences from diary.period_attendance 
    where id_customer = 1492 and id_diary_period = 172818 and id_user = 835603 t1;
    CROSS APPLY 
    (SELECT t1.total_presences /count(*) 
    from diary.lesson 
    where id_diary_period =  and id_customer =  and end_date < now();
    ) t2;

Answer 2

可能最简单的方法是使用 CTE：

WITH lesson_count AS (
   select count(*) as lessons
   from diary.lesson 
   where id_diary_period =  and id_customer =  and end_date < now()
)
select total_presences, total_presences/lessons
from diary.period_attendance, lesson_count
where id_customer = 1492 
  and id_diary_period = 172818 
  and id_user = 835603;

根据 total_presences 的类型，您可能必须将其转换为数字、实数或浮点数以避免整数运算。

如何将来自不同表的这两个查询合并为一个来计算百分比？

How to combine these two queries from different tables into one to calculate percentage?

sql

postgresql

querying