SQL - 显示是否在另一条记录后创建了一条记录

Question

我希望能够检查某个特定的记录（在本例中，“under”是在创建一个初始记录后创建的， “下”或空。

示例数据

Received Name Sub 
01-Jun  Mike Over 
01-Jun  John Over 
02-Jun  Dave Between 
03-Jun  Pete Over 
02-Jun  Mike Under 
03-Jun  Dave Under 

预期的效果

Received Name Sub  Sub2 
01-Jun  Mike Over Under 
01-Jun  John Over Null 
02-Jun  Dave Between Under 
03-Jun  Pete Over Null 

我从这个代码工作，但它很可怕的错误，我不能见树不见林了。

DECLARE @TM DATETIME; 
SET @TM = DATEADD(MONTH, DATEDIFF(MONTH, '19000101', GETDATE()), '19000101'); 

select 
t1.received, 
t1.name, 
t1.sub, 
t2.sub as sub2 

from 
dbo.tblOpen t1 
join dbo.tblOpen t2 on t1.name = t2.name 

where 
t1.closed >= DATEADD(MONTH, -1, @TM) 

请问我可以在这里得到一个正确的方向。

Answer 1

很多方法来处理这一个。

相关子查询应该工作：

SELECT t1.received, 
     t1.name, 
     t1.sub, 
     CASE WHEN EXISTS (SELECT * 
         FROM dbo.tblOpen t2 
         WHERE t2.Name = t1.Name 
         AND t2.received > t1.received 
         AND t2.sub = 'UNDER') 
      THEN 'UNDER' 
     END AS sub2 
FROM dbo.tblOpen t1 
WHERE t1.sub <> 'UNDER' 

还是一个LEFT JOIN：

SELECT t1.received, 
     t1.name, 
     t1.sub, 
     t2.sub as sub2 
FROM dbo.tblOpen t1 
LEFT JOIN dbo.tblOpen t2 ON t2.Name = t1.Name 
         AND t2.received > t1.received 
         AND t2.sub = 'UNDER' 
WHERE t1.sub <> 'UNDER' 

根据您的实际数据，事情可能会变得更加复杂。例如，对于任何给定的名称，'UNDER'是否可以多次出现？如果是这样，你只想看一次，或每次出现一次？无论如何，根据您的示例数据，这是一个很好的起点。