Question

I’ve got two tables in my MySQL database:

DisplayArticles

Id
CategoryId
ArticleId
IsAuto

Articles

Id
CategoryId
Title

Now what I need to do is to write MySQL query that will fill all positions where IsAuto set to true with newest articles from specified category (with the newest one on top position).

For example if I will have articles:

...

53 2 'Article that is older'
54 2 'Article about something funny'
55 6 'Article about something else'
56 2 'Article about something interesting'

And my display articles like:

1  2  12 false
2  2  42 true
3  2  41 true
4  2  23 false
5  2  25 false

Query invoked with with a parameter CategoryId = 2 should modify DisplayArticles table like this:

1  2  12 false
2  2  56 true
3  2  54 true
4  2  23 false
5  2  25 false

Hope this is clear and understandable.
Please help me to write this query.

Thanks for any response.

Answer 1

Following should do it.

The gist of it goes like this

• Use a subselect matching on CategoryID
• Select the latest ArticleID from Articles in this subselect excluding ArticleID‘s that are already assigned
• Add a WHERE clause only updating where IsAuto equals true.

Update statement

UPDATE DisplayArticles
SET    ArticleID = (SELECT MAX(ArticleID) 
                    FROM   Articles AS a                        
                    WHERE  a.CategoryID = CategoryID
                           AND ArticleID NOT IN (
                             SELECT ArticleID 
                             FROM   DisplayArticles da
                             WHERE  da.CategoryID = CategoryID 
                           )
                   )
WHERE  IsAuto = 'true'