ImageId (int)
VotesUp (int)
VotesDown (int)

ImageId (int)
UserId (int)
Score (int )   [-1,0,+1]

alter TRIGGER [dbo].[UpvotesChanged]
ON [dbo].[Upvotes]
FOR INSERT, UPDATE
AS
BEGIN
IF EXISTS( SELECT 1  FROM   DELETED ) --update
BEGIN

UPDATE imgs
SET    VotesUp = CASE

                    WHEN deleted.Score = 1 AND INSERTED.score =0      THEN ISNULL(VotesUp, 0) -1
                    WHEN deleted.Score = 1 AND INSERTED.score =-1      THEN ISNULL(VotesUp, 0) -1
                    WHEN deleted.Score = 0 AND INSERTED.score =1      THEN ISNULL(VotesUp, 0) +1
                    WHEN deleted.Score = -1 AND INSERTED.score =1      THEN ISNULL(VotesUp, 0) +1
                    ELSE ISNULL(VotesUp, 0)
                END
                ,
    VotesDown = CASE

                    WHEN deleted.Score = 0 AND INSERTED.score =-1      THEN ISNULL(VotesDown, 0) +1
                    WHEN deleted.Score = 1 AND INSERTED.score =-1      THEN ISNULL(VotesDown, 0) +1
                    WHEN deleted.Score = -1 AND INSERTED.score =1      THEN ISNULL(VotesDown, 0) -1
                    WHEN deleted.Score = -1 AND INSERTED.score =0      THEN ISNULL(VotesDown, 0) -1
                    ELSE ISNULL(VotesDown, 0)
                END
FROM   Images imgs
    JOIN DELETED
        ON  imgs.ImageId = deleted.ImageId
        JOIN INSERTED ON imgs.ImageId = INSERTED.ImageId


END
ELSE
--insert
BEGIN
UPDATE imgs
SET    VotesUp = CASE
                    WHEN INSERTED.Score = 1
                    THEN ISNULL(VotesUp, 0) + 1
                    ELSE VotesUp
                END,
    VotesDown = CASE
                    WHEN INSERTED.Score = -1
                        THEN ISNULL(VotesDown, 0) + 1
                    ELSE VotesDown
                END
FROM   Images imgs
    JOIN INSERTED
        ON  imgs.ImageId = INSERTED.ImageId
END
END