Join another table with multiple rows to another table's single result

Question

I currently select a single row (a post):

SELECT s.id AS id,s.date,s.title,s.views,s.image,s.width,s.description,u.id AS userId,u.username,u.display_name,u.avatar,
 (select count(*) from comments where item_id = s.id and type = 1) as numComments,
 (select count(*) from likes where item_id = s.id and type = 1) as numLikes,
 (select avg(value) from ratings where showcase_id = s.id) as average,
 (select count(*) from ratings where showcase_id = s.id) as total
FROM showcase AS s
INNER JOIN users AS u ON s.user_id = u.id
WHERE s.id = :id
LIMIT 5

Then get comments for that post in a separate query:

SELECT c.id as c_id,c.text,c.date,u.id as u_id,u.username,u.display_name,u.avatar
FROM comments as c
INNER JOIN users as u ON c.user_id = u.id
WHERE item_id = :item_id AND type = :type

:id and :item_id are the same. However, the comments return multiple rows whereas the first query returns one row - is there a way to join the comments to the first query or is the current way fine?

Answer 1

It really depends on your application.

If we are talking about a few records returned from a small or medium table, and if the query is executed just a few times a day, then it wouldn't matter much if:

1. you work with two record sets (two different queries are executed and then their results are put together);

2. you join the two queries, copying the post information for each record from the comments query;

3. you build a XML with the comments and join it to the record returned in the first query (the post record).

Another factor to take in consideration is whether the post and it's comments are displayed at the same time. If this is NOT the case and the comments are not visible at first and displayed only after some action like the click of a button, then you should chose the 1st option above, for performance reasons.

But if both the post information it's comments must be displayed at the same time, then you should chose one of the 3 options above. Which one is more of a personal favorite in modeling your application data structures and it's database access layer.

Now, if the volume of data may get huge, then you should dig a little deepen and run some simulations to find the query(ies) that give you the optimal performance.