Question

For a class project, a few others and I have decided to make a (very ugly) limited clone of StackOverflow. For this purpose, we’re working on one query:

Home Page: List all the questions, their scores (calculated from votes), and the user corresponding to their first revision, and the number of answers, sorted in date-descending order according to the last action on the question (where an action is an answer, an edit of an answer, or an edit of the question).

Now, we’ve gotten the entire thing figured out, except for how to represent tags on questions. We’re currently using a M-N mapping of tags to questions like this:

CREATE TABLE QuestionRevisions (
id INT IDENTITY NOT NULL,
question INT NOT NULL,
postDate DATETIME NOT NULL,
contents NTEXT NOT NULL,
creatingUser INT NOT NULL,
title NVARCHAR(200) NOT NULL,
PRIMARY KEY (id),
CONSTRAINT questionrev_fk_users FOREIGN KEY (creatingUser) REFERENCES
Users (id) ON DELETE CASCADE,
CONSTRAINT questionref_fk_questions FOREIGN KEY (question) REFERENCES
Questions (id) ON DELETE CASCADE
);

CREATE TABLE Tags (
id INT IDENTITY NOT NULL,
name NVARCHAR(45) NOT NULL,
PRIMARY KEY (id)
);

CREATE TABLE QuestionTags (
tag INT NOT NULL,
question INT NOT NULL,
PRIMARY KEY (tag, question),
CONSTRAINT qtags_fk_tags FOREIGN KEY (tag) REFERENCES Tags(id) ON
DELETE CASCADE,
CONSTRAINT qtags_fk_q FOREIGN KEY (question) REFERENCES Questions(id) ON
DELETE CASCADE
);

Now, for this query, if we just join to QuestionTags, then we’ll get the questions and titles over and over and over again. If we don’t, then we have an N query scenario, which is just as bad. Ideally, we’d have something where the result row would be:

+-------------+------------------+
| Other Stuff | Tags             |
+-------------+------------------+
| Blah Blah   | TagA, TagB, TagC |
+-------------+------------------+

Basically — for each row in the JOIN, do a string join on the resulting tags.

Is there a built in function or similar which can accomplish this in T-SQL?

Answer 1

Here’s one possible solution using recursive CTE:

The methods used are explained here

TSQL to set up the test data (I’m using table variables):

DECLARE @QuestionRevisions TABLE  ( 
id INT IDENTITY NOT NULL, 
question INT NOT NULL, 
postDate DATETIME NOT NULL, 
contents NTEXT NOT NULL, 
creatingUser INT NOT NULL, 
title NVARCHAR(200) NOT NULL)

DECLARE @Tags TABLE ( 
id INT IDENTITY NOT NULL, 
name NVARCHAR(45) NOT NULL
)

DECLARE @QuestionTags TABLE ( 
tag INT NOT NULL, 
question INT NOT NULL
)
INSERT INTO @QuestionRevisions 
(question,postDate,contents,creatingUser,title)
VALUES
(1,GETDATE(),'Contents 1',1,'TITLE 1')

INSERT INTO @QuestionRevisions 
(question,postDate,contents,creatingUser,title)
VALUES
(2,GETDATE(),'Contents 2',2,'TITLE 2')

INSERT INTO @Tags (name) VALUES ('Tag 1')
INSERT INTO @Tags (name) VALUES ('Tag 2')
INSERT INTO @Tags (name) VALUES ('Tag 3')
INSERT INTO @Tags (name) VALUES ('Tag 4')
INSERT INTO @Tags (name) VALUES ('Tag 5')
INSERT INTO @Tags (name) VALUES ('Tag 6')

INSERT INTO @QuestionTags (tag,question) VALUES (1,1)
INSERT INTO @QuestionTags (tag,question) VALUES (3,1)
INSERT INTO @QuestionTags (tag,question) VALUES (5,1)
INSERT INTO @QuestionTags (tag,question) VALUES (4,2)
INSERT INTO @QuestionTags (tag,question) VALUES (2,2)

Here’s the action part:

;WITH CTE ( id, taglist, tagid, [length] ) 
      AS (  SELECT question, CAST( '' AS VARCHAR(8000) ), 0, 0
            FROM @QuestionRevisions qr
            GROUP BY question
            UNION ALL
            SELECT qr.id
                ,  CAST(taglist + CASE WHEN [length] = 0 THEN '' ELSE ', ' END + t.name AS VARCHAR(8000) )
                ,  t.id
                ,  [length] + 1
            FROM CTE c 
            INNER JOIN @QuestionRevisions qr ON c.id = qr.question
            INNER JOIN @QuestionTags qt ON qr.question=qt.question
            INNER JOIN @Tags t ON t.id=qt.tag
            WHERE t.id > c.tagid )
SELECT id, taglist 
FROM ( SELECT id, taglist, RANK() OVER ( PARTITION BY id ORDER BY length DESC )
         FROM CTE ) D ( id, taglist, rank )
WHERE rank = 1;