減少查詢數量

Question

我有單獨的查詢，但我需要減少“否”，因此將它們合而為一

 select   count(applicant_id)  as registered from student_application where filter_status=0 AND
  select  count(applicant_id) as filer_select  from student_application where filter_status=1 AND 
   select  count(applicant_id) as filter_reject  from student_application where filter_status=2

但這顯示了一些錯誤

Answer 1

使用CASE表達式。

詢問

select 
count(case when filter_status = 0 then applicant_id else null end) as registered,
count(case when filter_status = 1 then applicant_id else null end) as filer_select,
count(case when filter_status = 2 then applicant_id else null end) as filer_reject
from student_application;

SQL小提琴

Answer 2

如果要查找子集而不是filter_status的所有可能值，也可以將group_by與where子句一起使用：

SELECT filter_status, COUNT(*)
FROM student_application 
WHERE filter_status in (0, 1, 2) 
GROUP BY filter_status;