Question

I’ve tried to create a new table relation but I seem to have gotten an error for which I’m not sure why. The first relation (one to many) I made was last week and it works good, maybe I forgot the correct procedure since then?

Simple explanation:
I have three tables – events, music_styles, venues

So far, I correctly linked events and venues in the way that one venue can have multiple events linked to it. I tried making a new table that has rows for each musical style and an ID that would be used in table ‘events’ so that I can link each event to a musical style id.

However this is the error I got:

SQL query:

ALTER TABLE events ADD FOREIGN KEY ( MUSIC_STYLE_ID ) REFERENCES
nightl7_complete.music_styles ( MUSIC_ID ) ON DELETE NO ACTION
ON UPDATE NO ACTION ;

MySQL said: Documentation

1452 – Cannot add or update a child row: a foreign key constraint

fails (nightl7_complete/#sql-2721_c0dcfd, CONSTRAINT
#sql-2721_c0dcfd_ibfk_2 FOREIGN KEY (MUSIC_STYLE_ID) REFERENCES
music_styles (MUSIC_ID) ON DELETE NO ACTION ON UPDATE NO ACTION)
Documentation Error ALTER TABLE events ADD FOREIGN KEY (
MUSIC_STYLE_ID ) REFERENCES nightl7_complete.music_styles (
MUSIC_ID ) ON DELETE NO ACTION ON UPDATE NO ACTION ;

And this is the procedure I did 🙂

I uploaded a full sized image here

Thanks everyone 🙂

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Edit:

So I did this:

$query = 'SELECT e.* FROM events e '.
    'LEFT JOIN nightl7_complete.music_styles ms ON ms.ID = e.MUSIC_STYLE_ID'.
    'WHERE ms.id IS NULL';
$result = mysql_query($query);
if (!$result) {
  die('Invalid query: ' . mysql_error());
}

and got this:

Invalid query: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘ms.id IS NULL’ at line 1

Answer 1

Don’t you want to link MUSIC_STYLE_ID to ID, not MUSIC_ID?

Also, you need to either pre-populate MUSIC_STYLE_ID with valid ids from music_styles, or set them to null before creating the foreign key.

To verify this, try:

SELECT e.*
  FROM events e
  LEFT JOIN nightl7_complete.music_styles ms ON ms.ID = e.MUSIC_STYLE_ID
  WHERE ms.id IS NULL;

If that query returns any records, those are entries in events that don’t contain corresponding records in music_styles