如何将这两个来自不同表的查询合并为一个来计算百分比？答案

【问题标题】：How to combine these two queries from different tables into one to calculate percentage?如何将这两个来自不同表的查询合并为一个来计算百分比？
【发布时间】：2020-02-19 16:53:13
【问题描述】：

我有以下查询，该期间有学生出勤：

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

我有同一时期的课数。

select count(*) from diary.lesson where id_diary_period = $1 and id_customer = $2 and end_date < now();

我想将 total_presences 除以课程数来获得学生的出勤率。

如何在单个查询中做到这一点？

【问题讨论】：

标签： sql postgresql querying

【解决方案1】：

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 = $1 and id_customer = $2 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，您可能必须将其强制转换为数字、实数或浮点数以避免整数数学运算。

【讨论】：

【解决方案2】：

你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 = $1 and id_customer = $2 and end_date < now();
    ) t2;

【讨论】：