检查包含项目以逗号分隔的列表作为SQL Server中的查询结果

Question

I have a Post table in SQL Server and for performance reason, this table has a column(UsersWhoLiked) in the form of comma separated values that specify which UserIds that liked a Post as following.

Id | Subject |  UsersWhoLiked
---|---------|-----------
1  | red     |  u1,u2
2  | blue    |  u2, u3,u4,u5
3  | green   |  
4  | gray    |  u3, u4, u7

I want to write a query that gets UserId as a parameter and return List of Post as a result so that in the result, every post have a column that specifies the user liked or not the post.

Example: For UserId: u3

I expect that:

Id | Subject |  Liked
---|---------|-----------
1  | red     |  0
2  | blue    |  1
3  | green   |  0
4  | gray    |  1

Query:

SELECT Id, Subject FROM Post

But I don't know how to add Liked column to the Select statement

Answer 1

You can use the following:

SELECT Id, Subject, 
    IIF(CHARINDEX('u3,', REPLACE(CONCAT(UsersWhoLiked, ','), ' ', '')) > 0, 1, 0) AS Liked 
FROM Post

demo: http://sqlfiddle.com/#!18/3a588/1/0

---

You should use a second table to store the likes:

CREATE TABLE PostLikes (
    PostId INT FOREIGN KEY REFERENCES Post(Id),
    UserId INT FOREIGN KEY REFERENCES Users(Id),
    CONSTRAINT PK_PostLikes PRIMARY KEY (PostId, UserId)
);

So you can use the following SELECT to get the expected information:

SELECT Id, Subject, IIF(ISNULL(PostLikes.UserId, 0) > 0, 1, 0) AS Liked
FROM Post LEFT JOIN PostLikes ON Post.Id = PostLikes.PostId AND PostLikes.UserId = 3

demo: http://sqlfiddle.com/#!18/176ce/4/0

Answer 2

You can try the below:

DECLARE @id varchar(10)='u3'
SELECT Id, Subject, 
CASE WHEN (',' + UsersWhoLiked + ',') LIKE '%,'+@id+',%' THEN 1 ELSE 0 END Liked
FROM Post

Answer 3

I don't how storing comma separated values improves performance, but this is bad idea. In your case you should be careful with substrings. Such as, if you are looking for u3 , it should not match u31 . So put , before and after each value in UsersWhoLiked column

declare @userId varchar(10) = 'u3'
select 
    Id, Subject
    , Liked = isnull(sign(charindex(',' + @userId + ',', ',' + UsersWhoLiked + ',')), 0)
from Post

Answer 4

select *, case PATINDEX('%u3%', userwholikes) --dont use u3(comma), only use u3
when 0 then                                   --because u3 becomes last one
0
else 1
end likes
from yourtable

i give this, by my understanding..