仅在TSQL中满足条件时才选择列

Question

I have two tables. Customers and Images.

I join these and i select an Image Id from the Images table ONLY if the image is the profilepicture.

However if a user has several pictures but only on of the pictures is the profile picture i still get back several rows with the same user.

my code:

SELECT DISTINCT c.Id, c.Name, c.LastName, c.Email, c.MobilePhoneNumber,
(SELECT CASE WHEN im.Isprofile = 1 THEN im.Id END) AS ImageId,
 im.ContentType 
   FROM Customers c
JOIN Images im ON im.CustomerId = c.Id

The "CASE WHEN" is obviously wrong and this feels overly simple thing to do but I have been at this for a while and can't figure it out.

EDIT: I get this result:

I only want one row here since ImageId is null

Answer 1

SELECT DISTINCT c.Id,
  c.Name, 
  c.LastName,
   c.Email,
  c.MobilePhoneNumber,
 (im.Id) AS ImageId, im.ContentType FROM Customers c
LEFT JOIN Images im ON im.CustomerId = c.Id and im.Isprofile = 1

Hope this helps

Answer 2

Use OUTER APPLY :

SELECT c.Id, 
       c.Name, 
       c.LastName, 
       c.Email, 
       c.MobilePhoneNumber, 
       im.ID AS ImageID, 
       im.ContentType 
FROM Customers c
OUTER APPLY(SELECT TOP 1 ID, ContentType FROM Images im 
            WHERE im.CustomerId = c.Id AND im.Isprofile = 1) im

Answer 3

It's up to your implementation, but it sounds like you might benefit in the long-run from a view for this. Here's the equivalent to what I'd do there, but with a CTE in its place.

WITH ProfilePictures AS (
    SELECT ID, ContentType, CustomerId
    FROM Images i
    WHERE i.IsProfile = 1
)

SELECT c.Id, c.Name, c.LastName, c.Email, c.MobilePhoneNumber, im.Id AS ImageId, im.ContentType
FROM Customers c
    LEFT OUTER JOIN ProfilePictures im ON im.CustomerId = c.Id

This implementation assumes that every customer has exactly zero or one images marked with IsProfile = 1 .

Just to be clear, this is overkill if you're only looking at this logic once or twice. I'm just figuring tying customers to their pictures might be something you do a lot, in which case abstracting that logic out could come in handy.

---

For what it's worth, if my interpretation is correct, and certainly this is up to you, I'd be tempted to drop the IsProfile bit, and add a field on Customers for ProfileImageId with an appropriate foreign key. Then the join will be extremely straight-forward, and you won't have to worry about maintaining that logic.

Answer 4

Try It :

SELECT 
    DISTINCT cust.Id
        ,cust.Name
        ,cust.LastName
        ,cust.Email
        ,cust.MobilePhoneNumber
        ,img.Id AS ImageId
        ,img.ContentType 
FROM Customers as cust
LEFT OUTER JOIN Images as img ON img.CustomerId = cust.Id and img.Isprofile = 1