How can I efficiently query 'Like' data from MySQL?

Question

I am developing a social network iOS app using a PHP backend with a MySQL database. I have 3 tables in my database to store Posts, Comments and Likes.

Currently, the app requests up to 50 posts at a time which includes various specific data. I do this using a SELECT query from MySQL. My 'Likes' are stored in the 'Likes table' as 1 Like per row. This includes their username and which post they 'liked'.

When the app is refreshed, I need to gather the 'Likes' data on the posts that have just been loaded. My problem is that I do not know the best way to query MySQL to return all associated likes with that user. This means that when I load some posts, I want the user to see which posts they have already liked.

Here is a visual description of my problem:

Please understand that up to 50 of these posts will be shown at one time.

Answer 1

There is several way to do this and depending on your project you might want to optimise my answer.

I assume your Likes table has a structure of (int) id, (int) user_id, (int) post_id .

To answer directly to your question (about doing a query only for the refresh), the next MySQL query will give you the list of post_id liked by the user. You need to replace %user_id% by the id of the user and %post_ids% by the list of the 50 posts ids, separated by a comma:

SELECT post_id
FROM Likes
WHERE user_id = %user_id% AND
      post_id IN (%post_ids%)

However, if the refresh implies the whole page (and not only an AJAX request or so on) I would consider implementing this directly in the query getting the posts. For example :

SELECT p.id, p.content, p...., l.id AS `Liked`
FROM Posts AS p
LEFT JOIN Likes AS l
       ON p.id = l.post_id AND
          l.user_id = %user_id%  
WHERE ... 

Please also consider adding indexes on the right columns.

Answer 2

As promised, my solution is here:

SELECT `posts`.`postnumber`, `posts`.`message`, `likes`.`didLike`
FROM `posts`
LEFT JOIN `likes`
ON `posts`.`postnumber`=`likes`.`post_id`
AND `likes`.`user_id`='USERNAME';