Hope you can find a solution to this. I've searched the site and tried 'self-joins' but can't get this to work....
By submitting your email address, you agree to receive emails regarding relevant topic offers from TechTarget and its partners. You can withdraw your consent at any time. Contact TechTarget at 275 Grove Street, Newton, MA.
The current solution we're running requires 15 queries to get the necessary result! I have three MySQL tables:
- One stores articles with publish dates and other details.
- One stores industries.
- One serves as a join for the two, where an article is linked to one or more industries using the article IDs and industry IDs.
I want to display the latest two articles from each industry. Can this be done in one or maybe two queries?
It can be done in one. This is an example of the "Latest X for each Y" common SQL question. See SQL FAQ: Common SQL Questions, part 3 (5 July, 2007). However, this one's a bit trickier since it involves a many-to-many join.
select I.industry , A.title , A.pubdate from industries as I inner join industryarticles as IA on IA.industry_ID = I.ID inner join articles as A on A.ID = IA.article_ID and 2 > ( select count(*) from industryarticles as IA2 inner join articles as A2 on A2.ID = IA2.article_ID where IA2.industry_ID = I.ID and A2.pubdate > A.pubdate ) order by I.industry , A.pubdate desc
The subquery—which is correlated to the outer query using the I correlation variable to link industries and the A correlation variable to link articles—counts the number of articles in the same industry which have a later date than the article in the outer query. This count must be less than the number 2. In other words, it must be 1 or 0. If the article being considered in the outer query has only 1 or 0 articles in the same industry with a later date, then it has to be among the latest two for that industry.
Dig Deeper on Oracle and SQL
Related Q&A from Rudy Limeback
Read an example of an SQL case expression from our SQL expert Rudy Limeback.continue reading
Read about the Mimer Validator, a tool used to verify your SQL code, in this tip from SQL expert Rudy Limeback.continue reading
Read SQL expert Rudy Limeback's advice for counting combinations in a table with SQL's GROUP BY clausecontinue reading
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.