MySQL复杂选择查询表5

Question

I have 5 table:

1. mp3s
2. albums
3. remixes
4. users
5. likes

likes table:

╔════════╦══════════╦═════════╦═════════════╦═════════════════════╗
║ id     ║ user_id  ║ type_id ║ target_id   ║ like_date           ║
╠════════╬══════════╬═════════╬═════════════╬═════════════════════╣
║ 1      ║ 1        ║ 1       ║ 1049        ║ 2016-05-23 19:50:41 ║
╠════════╬══════════╬═════════╬═════════════╬═════════════════════╣
║ 2      ║ 2        ║ 2       ║ 457         ║ 2016-01-09 19:50:42 ║
╠════════╬══════════╬═════════╬═════════════╬═════════════════════╣
║ 3      ║ 2        ║ 3       ║ 457         ║ 2016-01-09 19:50:42 ║
╠════════╬══════════╬═════════╬═════════════╬═════════════════════╣
║ 4      ║ 2        ║ 1       ║ 457         ║ 2016-01-09 19:50:42 ║
╠════════╬══════════╬═════════╬═════════════╬═════════════════════╣
║ 5      ║ 3        ║ 3       ║ 4955        ║ 2016-06-12 19:50:41 ║
╚════════╩══════════╩═════════╩═════════════╩═════════════════════╝

type_id columns:

• 1--> mp3s
• 2--> albums
• 3--> remixes

i need this query like:

select col1, col2, col3
from likes, mp3s, albums, remixes
     where likes.user_id == 2

          if (likes.type_id == 1)
          select col1, col2, col3
          from mp3s
          where likes.target_id == mp3s.id

          union

          if (likes.type_id == 2)
          select col1, col2, col3
          from albums
          where likes.target_id == albums.id

          union

          if (likes.type_id == 3)
          select col1, col2, col3
          from remixes
          where likes.target_id == remixes.id

 order by likes.like_date desc
 limit 0,20

thanks.

Answer 1

You'll need to use unions and joins

This selects all of the valid mp3 rows.

select 
    col1, col2, col3
from 
    likes
inner join
    mp3s
on likes.target_id = mp3s.id
where likes.type_id = 1 -- type is mp3

Now, so I dont do all the work for you -- create two more queries to get just the remixes and just the albums -- and then join them, maybe with a union?

Answer 2

Try this:

SELECT IF(likes.type_id = 1, mp3s.col1, IF(likes.type_id = 2, albums.col1, remixes.col1)) col1,
       IF(likes.type_id = 1, mp3s.col2, IF(likes.type_id = 2, albums.col2, remixes.col2)) col2,
       IF(likes.type_id = 1, mp3s.col3, IF(likes.type_id = 2, albums.col3, remixes.col3)) col3
FROM
    likes
LEFT JOIN mp3s
    ON likes.type_id = 1 AND likes.target_id == mp3s.id
LEFT JOIN albums
    ON likes.type_id = 2 AND likes.target_id == albums.id
LEFT JOIN remixes
    ON likes.type_id = 3 AND likes.target_id == remixes.id
WHERE likes.user_id == 2
ORDER BY likes.type_id, likes.like_date desc