如何使用连接查询从多个表中获取计数（id）

Question

我想根据多个表中的id显示计数。对于两个表，它工作正常，但对于三个表，它不显示数据

这是我对三个表无法正常工作的查询

select r.req_id
     , r.no_of_positions
     , count(j.cand_id) as no_of_closure
     , count(cis.cand_id) 
from requirement r 
join joined_candidates j 
  on r.req_id=j.req_id
join candidate_interview_schedule cis 
  on cis.req_id=r.req_id 
where cis.interview_status='Interview Scheduled' 
group by r.req_id, r.no_of_positions;

Answer 1

1. 更改为左连接，表格中存在值不存在
2. 更改计数以使用窗口功能，因此连接不会人为地夸大计数
3. 将where子句移动到连接条件，就像在左连接上一样，它会否定空值，使其像内连接一样运行。

..也许...

SELECt r.req_id
     , r.no_of_positions
     , count(j.cand_id) over (partition by J.cand_ID) as no_of_closure
     , count(cis.cand_id) over (partition by cis.cand_id) as no_of_CIS_CNT
FROM  requirement r 
LEFT join joined_candidates j 
  on r.req_id=j.req_id
LEFT join candidate_interview_schedule cis 
  on cis.req_id=r.req_id 
 and  cis.interview_status='Interview Scheduled' 
GROUP BY  r.req_id, r.no_of_positions;

或者......（如果我可以假设j.cand_ID和cis.cand_ID是唯一的）也可以消除由于1：M连接引起的人工计数增加

SELECt r.req_id
     , r.no_of_positions
     , count(distinct j.cand_id) as no_of_closure
     , count(distinct cis.cand_id) as no_of_CIS_CNT
FROM  requirement r 
LEFT join joined_candidates j 
  on r.req_id=j.req_id
LEFT join candidate_interview_schedule cis 
  on cis.req_id=r.req_id 
 and  cis.interview_status='Interview Scheduled' 
GROUP BY  r.req_id, r.no_of_positions;