Question

My objective is to write a query that will return all the Categories with 3 flag fields, like this:

ID | En | Ru | Fr

The language flags must be ON or OFF according to if is any Lesson record associated with the category.

My current statement is this:

SELECT c.ID,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 1) THEN 1 ELSE 0 END) AS En,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 2) THEN 1 ELSE 0 END) AS Ru,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 3) THEN 1 ELSE 0 END) AS Fr
FROM LessonCategories AS c

The problem is that this query is VERY slow, since the Lessons table has more than 60,000 records, and i’m running over it 3 times.

I’m looking for a way to make this query more efficient.
I had a thought to use grouping on join between Categories and lessons but i dont know exactly how and if it is even possible.

The pseudo-code for more fast query is:

SELECT c.[ID], 
    COUNT(l.Language_Id = 1) > 0 AS En
    COUNT(l.Language_Id = 2) > 0 AS Ru
    COUNT(l.Language_Id = 3) > 0 AS Fr
FROM CategoryTreeView AS c
INNER JOIN Lessons AS l ON l.Category_Id = c.ID
GROUP BY c.[ID]

It it possible to express this using valid t-sql?
Or is any better way to handle this kind of query?

P.S. If it helps, i don’t care to get one bitwise flag field instead of the 3 language fields.

Thanks.

Answer 1

Well you have just to combine your two queries 😉

SELECT c.ID, 
       SUM(case when l.Language_Id = 1 then 1 else 0) AS En
       SUM(case when l.Language_Id = 2 then 1 else 0) AS Ru
       SUM(case when l.Language_Id = 3 then 1 else 0) AS Fr
FROM CategoryTreeView AS c
INNER JOIN Lessons AS l ON l.Category_Id = c.ID
GROUP BY c.ID