how to select first two rows from each group - mysql

Question

Hi We have 3 Table of a music which is something like this in MySql :
1st Table :
the first table is for playlist table where music playlist is exist.
playlistId playlistTitle categoryId
1 hello 0
2 wow 0
3 wi-fi 0
4 awesome 0
5 sixer 1
6 four 1
7 boundary 2
2nd Table :
2nd table is for `songRelation table where every playlist is associated with thier song
playlistId songId
1 4
1 3
1 43
1 57
1 98
2 56
2 67
2 90
2 78
3 98
3 78
3 89
43 90
3rd Table :
the 3rd table is for song table where song detail exist
songId songTitle
4 hello
3 real hero
43 singalone
57 awesom
98 really
78 sakaka
98 shikwa
89 moha
90 hello2
67 Sneh
actually i want to get result something like this :
playlistId songId categoryId songTitle
1 4 0 hello
1 3 0 real hero
2 56 0 singalone
2 67 0 Sneh
3 78 0 sakaka
3 98 0 Shikwa
where the every playlistId will be with their first 2 songId and with their categoryId and also with songTitle.

Answer 1

You can use variables:
SELECT playlistId, songId, categoryId, songTitle
FROM (
SELECT p.playlistId, s.songId, p.categoryId, s.songTitle,
#r := IF (#pid = p.playlistId,
IF (#pid := p.playlistId, #r+1, #r+1),
IF (#pid := p.playlistId, 1, 1)) AS rn
FROM playlist AS p
CROSS JOIN (SELECT #r:=0, #pid:=0) AS vars
INNER JOIN songRelation AS sr ON p.playlistId = sr.playlistId
INNER JOIN song AS s ON sr.songid = s.songid
ORDER BY p.playlistId, s.songId ) AS t
WHERE t.rn <= 2
Variable #r is used to enumerate records within each playlistId slice. Using this in an outer query, we can easily get 2 records per playlistId slice.
Demo here

Answer 2

you can do this using JOIN, try this code:-
SELECT playlist.playlistid, songRelation.songId, song.songTitle
FROM playlist JOIN songRelation JOIN song
WHERE playlist.playlistId=songRelation.playlistId
AND songRelation.songId=song.songId LIMIT 2
you can add the category table and use the same way to get the result table you need, you can also save this query as a virtual table VIEW by writing this code before the code:-
CREATE VIEW myView AS
Edit:-
SELECT playlist.playlistId from playlist INNER JOIN( SELECT playlist.playlistId, songRelation.songId, song.songTitle
FROM playlist JOIN songRelation JOIN song
WHERE playlist.playlistId=songRelation.playlistId
AND songRelation.songId=song.songId
GROUP BY playlist.playlistId LIMIT 2)
WHERE playlist.playlistId=songRelation.playlistId

Answer 3

Please try it, as it is not tested with data, so if you get any issue then create an sqlfiddle so that I can solve the issue.
SELECT x.*
FROM (SELECT pl.playlistid, sng.songId, sng.songTitle,
CASE
WHEN #category != pl.playlistid THEN #rownum := 1
ELSE #rownum := #rownum + 1
END AS rank,
#category := pl.playlistid AS play_list
FROM playlist AS pl
JOIN songRelation AS sr ON sr.playlistid=pl.playlistid
JOIN song AS sng ON sng.songid=sr.songid
JOIN (SELECT #rownum := NULL, #category := '') r
ORDER BY pl.playlistid,sng.songid) X
WHERE x.rank<=2