如何在 SQL 中将所有行放在同一级别？

Question

尝试将此查询用于客户要求贷款的过程：

select loan_id,
CASE WHEN status = 'document_sent' then date ELSE NULL END as document_sent,
CASE WHEN status = 'document_rejected' then date ELSE NULL END as document_rejected
from table
and status in ('document_sent', 'document_rejected')
order by 2 asc, 3 asc

结果如下：

loan_id	document_sent	doc_rejected
123	2021-03-01 14:52
123	2021-03-01 14:57
123	2021-03-01 15:33
123		2021-03-01 14:54
123		2021-03-01 15:00
123		2021-03-01 15:39

我想要这样的东西：

loan_id	document_sent	doc_rejected
123	2021-03-01 14:52	2021-03-01 14:54
123	2021-03-01 14:57	2021-03-01 15:00
123	2021-03-01 15:33	2021-03-01 15:39

有可能吗？谢谢

Answer 1

如果我们不对每种记录的缺失/存在做出任何假设：

select 
 coalesce(a.loan_id,b.loan_id) as loan_id
 , ds.date as document_sent
 , dr.date as document_rejected
from 
   (select * from tbl where status='document_sent') ds
   full outer join
   (select * from tbl where status='document_rejected') dr
   on ds.loan_id=dr.loan_id
   

如果您知道始终存在一种或另一种状态，则可以稍微简化一下（可以转换为left join），或者如果预计两条记录都存在，则可以简化为：

select 
  a.loan_id
 , ds.date as document_sent
 , dr.date as document_rejected
from 
   tbl ds
   inner join
   tbl dr
   on ds.loan_id=dr.loan_id
   and ds.status='document_sent'
   and dr.status='document_rejected'

如果 `full external join 不可用：

select 
 coalesce(a.loan_id,b.loan_id) as loan_id
 , ds.date as document_sent
 , dr.date as document_rejected
from 
   (select * from tbl where status='document_sent') ds
   left join
   (select * from tbl where status='document_rejected') dr
   on ds.loan_id=dr.loan_id
union all
select 
 coalesce(a.loan_id,b.loan_id) as loan_id
 , ds.date as document_sent
 , dr.date as document_rejected
from 
   (select * from tbl where status='document_sent') ds
   right join
   (select * from tbl where status='document_rejected') dr
   on ds.loan_id=dr.loan_id
where ds.loan_id is null

感谢@nbk 指出。

您也可以使用 MIN/MAX（虽然我们不想找到任何东西的最小值/最大值，但我们希望利用它们的行为 1.它们接受日期，2.它们忽略空值，3.它们与GROUP BY 一起使用，将行组合在一起）：

select 
  loan_id
 , min(case when status='document_sent' 
            then ds.date else NULL end) as document_sent
 , min(case when status='document_rejected' 
            then ds.date else NULL end) as document_rejected
from 
   tbl
group by id