有关优化联接查询的建议

Question

I have 3 tables : videos , categories , video_categories .

In videos , I have id, title, and other fields. In categories , I have id and name. In video_categories , I have id, video_id, and category_id.

One video can have multiple categories. So the video_categories table will be something like this.

id   video_id  category_id
1    1         1
2    1         2
3    1         3

If I want to have a list of videos and display their categories, which would be preferred?

1. Via PHP, call 1 query to get all videos, then loop on that to query to get each video's categories, and another query to get the category name. This will be really slow if the table is huge, right?

2. Via MySQL joins (need help on this). If I left join videos to video_categories, there will be 3 results of the same video_id. I can use GROUP BY or SELECT DISTINCT to get unique result, but how can I now get the categories' names?

My expected result will be something like this:

id title    categories
1  Video1   pop, rock, jazz

Answer 1

For option 2, use GROUP_CONCAT . It will be ok

SELECT v.id, v.title, GROUP_CONCAT(c.name)
FROM videos v
INNER JOIN video_categories vc ON vc.video_id = v.id
INNER JOIN categories c ON vc.category_id = c.id
GROUP BY v.id, v.title

For Group_Concat() function , is the default separator. That's why I don't use it here.

Answer 2

Guessing the names of your other columns..

SELECT v.video_id, v.title, GROUP_CONCAT(c.category_name SEPARATOR ', ')
FROM videos v
LEFT JOIN video_categories vc ON vc.video_id = v.video_id
LEFT JOIN categories c ON c.category_id = vc.category_id
GROUP BY v.video_id, v.title