Question

First off, I am not very good with MySQL yet, so go easy on me. I am not even sure how to word my question.

I have two tables; one table contains information about links, the other ratings of each link made by users. Essentially, I just need to be able to order the links table by rating average.

TABLES:

links

• link_id PK int autoincrement
• url varchar
• title varchar
• description text
• rating decimal

link_ratings

• link_rating_id PK int autoincrement
• link_id FK int
• user_id FK int
• rating decimal

What I need is when someone adds a link rating to a link (new link_ratings entry) or updates their current rating, all of the ratings for that link are averaged and updated on the rating column in links. OR Can I scrap the rating column in the links table entirely and use a JOIN?

If so, how would I go about doing the JOIN. If not, how do I make TRIGGER that would accomplish this for me?

I haven’t been able to find much information, most likely due to my lack of SQL knowledge, of how to even start this.

Any help is appreciated. Thanks.

Answer 1

You don’t need to persist the rating average in the links table. I am calculating it below, and this should perform reasonably well:

select l.link_id,
    l.url,
    l.title,
    l.description,
    lra.AvgRating
from links l
left outer join (
    select link_id, avg(rating) as AvgRating
    from link_ratings
    group by link_id
) lra on l.link_id = lra.link_id
order by lra.AvgRating desc