SELECT DISTINCT有兩列

Question

我需要區分SenderId和RecipientId 。

所以我這樣做：

SELECT DISTINCT M.SenderId, R.StudentId as RecipientId
FROM Message M (nolock) INNER JOIN Recipient R (nolock) ON M.Id=R.MessageId 
GROUP BY M.SenderId, R.StudentId
HAVING StudentId=1 OR SenderId=1

這項工作可行，但我在那里也需要M.Text字段，但是沒有與眾不同的 。 所以我添加了這個：

GROUP BY M.SenderId, R.StudentId, M.Text

但這是行不通的。

Answer 1

這里有一些選擇； 從最適合您要求的措詞中不確定，但是懷疑有人會...

--selects unique combination of sender, recipient and text
--meaning the combo of 3 is unique, but within that combo values
--in each individual column may be repeated

SELECT DISTINCT M.SenderId
, R.StudentId as RecipientId
, M.Text
FROM Message M (nolock) 
INNER JOIN Recipient R (nolock) ON R.MessageId = M.Id
where StudentId=1 
or SenderId=1

要么

--returns all unique combos of SenderId and RecipientId
--along with a single corresponding Text field
--max() is just an arbitrary aggregate function to ensure we only
--get 1 result for M.Text

SELECT M.SenderId
, R.StudentId as RecipientId
, max(M.Text)
FROM Message M (nolock) 
INNER JOIN Recipient R (nolock) ON R.MessageId = M.Id
where StudentId=1 
or SenderId=1
group bu M.SenderId
, R.StudentId 

Answer 2

如果我已正確理解您的問題，則它將對您想要的內容進行分組，並區分SenderId和StudentId：

SELECT M.SenderId, R.StudentId as RecipientId, M.Text
FROM Message M (nolock) INNER JOIN Recipient R (nolock) ON M.Id=R.MessageId 
GROUP BY M.SenderId, R.StudentId, M.Text
HAVING COUNT(StudentId) = 1 OR COUNT(SenderId) = 1