mysql 左连接和内连接 3 个表

Question

I have 3 tables: oc_artists, oc_songs, oc_songs_tags

I can select 2 tables with this code:

SELECT * FROM `oc_songs` LEFT JOIN oc_songs_tags ON oc_songs.song_id=oc_songs_tags.song_id 
WHERE oc_songs_tags.song_tag IS NULL

Now I need to select data from oc_artists... I tried this questions answer: MySQL LEFT JOIN 3 tables

And here is my code:

SELECT * FROM oc_songs
LEFT JOIN oc_artists
    INNER JOIN oc_songs_tags
    ON oc_songs.song_artist_id = oc_artists.artist_id
ON oc_songs_tags.song_id = oc_songs.song_id

But I am getting this error

8 errors were found during analysis.

Unrecognized keyword. (near "ON" at position 131)
Unexpected token. (near "oc_songs_tags" at position 134)
Unexpected token. (near "." at position 147)
Unexpected token. (near "song_id" at position 148)
Unexpected token. (near "=" at position 156)
Unexpected token. (near "oc_songs" at position 158)
Unexpected token. (near "." at position 166)
Unexpected token. (near "song_id" at position 167)

Answer 1

Your syntax is wrong. Join syntax is:

[JOIN TYPE] JOIN [TABLE B] ON [TABLE A].[COLUMN] = [TABLE B].[COLUMN]

So that would mean:

SELECT * FROM oc_songs
 LEFT JOIN oc_artists ON oc_songs.song_artist_id = oc_artists.artist_id
 INNER JOIN oc_songs_tags ON oc_songs.song_id = oc_songs_tags.song_id

The simple way to understand JOINS is that they are ALWAYS a conjunction between the primary table listed in FROM and the table you are JOINING. So think of the JOIN clause as a list of conjunctions.

So to join TableA with TableB, TableC, and TableD you might have something like:

SELECT * FROM TableA
  JOIN TableB on TableA.tableb_id = TableB.id  -- this is first join
  JOIN TableC on TableA.tablec_id = TableC.id  -- this is second join
  JOIN TableD on TableA.tabled_id = TableD.id  -- this is third join

Answer 2

Wrong position of ON clause

SELECT * FROM oc_songs
LEFT JOIN oc_artists ON oc_songs.song_artist_id = oc_artists.artist_id
INNER JOIN oc_songs_tags  ON oc_songs_tags.song_id = oc_songs.song_id

Answer 3

Example tables:

DROP TABLE IF EXISTS `oc_songs` ;
DROP TABLE IF EXISTS `oc_songs_tags` ;
DROP TABLE IF EXISTS `oc_artists` ;

CREATE TABLE `oc_songs` (
  `song_id` int(10) unsigned NOT NULL auto_increment,
  `song_name` varchar(255) NOT NULL,
  `song_artist_id`  int(10),
  PRIMARY KEY  (`song_id`),
  foreign key (`song_artist_id`) references oc_artists(`artist_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

CREATE TABLE `oc_songs_tags` (
  `song_tag_id` int(10) unsigned NOT NULL auto_increment,
  `song_tag` varchar(255) NOT NULL,
  `song_id` int(10),
  PRIMARY KEY  (`song_tag_id`),
  foreign key (`song_id`) references oc_songs(`song_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

CREATE TABLE `oc_artists` (
  `artist_id` int(10) unsigned NOT NULL auto_increment,
  `artist_age` int(4),
  PRIMARY KEY  (`artist_id`)

) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

Right select:

SELECT * FROM oc_songs
 LEFT JOIN oc_artists ON 
    oc_songs.song_artist_id = oc_artists.artist_id
 INNER JOIN oc_songs_tags ON 
    oc_songs.song_id = oc_songs_tags.song_id;