多次删除，一张表为空时出现未知表错误

Question

I delete from two tables with primary entry.

• Both tables use to store image id.

• One for display image and other is to save in favourite.

• When both tables has entry, multi delete query works fine.

But:

• When favourite table is empty, query doesn't work for one table and show error as favourite is unknown table.

• Do you have any experiences multi delete from two tables (when one is empty)?

I tried in various ways but same errors:

$sql = "DELETE FROM stat, s_images, favourites USING stat, s_images,
        favourites WHERE favourites.image=stat.id AND stat.id=s_images.id AND
        stat.id=" . $this->id;

$sql = "DELETE stat, s_images,favourites FROM stat, s_images,
        favourites WHERE favourites.image=stat.id AND stat.id=s_images.id AND
        stat.id=" . $this->id;

Answer 1

Try using an explicit inner join

DELETE stat, s_images, favourites
FROM stat
INNER JOIN s_images
ON s_images.id = stat.id
INNER JOIN favourites
ON favourites.image = stat.id
WHERE stat.id = $this->id;

This will cause nothing to be deleted when a match is not found in any of the joins.

Example: http://sqlfiddle.com/#!9/580be/2

Otherwise you may need to perform individual queries or utilize Foreign Key Constraints to Cascade on delete.

You could also ignore the errors, but this is bad practice.

DELETE IGNORE stat, s_images, favourites
FROM stat
LEFT JOIN s_images
ON s_images.id = stat.id
LEFT JOIN favourites
ON favourites.image = stat.id
WHERE stat.id = $this->id;

---

UPDATE with Foreign Keys

Add constraint for s_images.id + stat.id

ALTER TABLE `stat`
    ADD CONSTRAINT `FK_stat_s_images` FOREIGN KEY (`id`) REFERENCES `s_images` (`id`) ON UPDATE CASCADE ON DELETE CASCADE;

Add constraint for s_images.id + favourites.image

ALTER TABLE `favourites`
    ADD CONSTRAINT `FK_favourites_s_images` FOREIGN KEY (`image`) REFERENCES `s_images` (`id`) ON UPDATE CASCADE ON DELETE CASCADE;

Then all you need to do is execute the query to delete the images, MySQL will ensure your records are managed appropriately, and will aid in optimizations.

$sql = "DELETE FROM s_images
WHERE s_images.id = '" . $this->id . "'";

---

Special Considerations

1. Tables must use the InnoDB engine as foreign keys do not work with MyISAM.

ALTER TABLE `stat` ENGINE=InnoDB;
ALTER TABLE `favourites` ENGINE=InnoDB;
ALTER TABLE `s_images` ENGINE=InnoDB;

2. INSERT and UPDATE must use proper ordering, meaning you can not create an association on favourites.image = 1 when the record for s_images.id = 1 does not exist. The record s_images.id = 1 would need to be created prior to the favourites record.

3. You will not be able to add the FOREIGN KEY constraints when an id is referenced that does not exist. You can override this behavior by using

SET FOREIGN_KEY_CHECKS=0;
 ALTER TABLE `stat`
        ADD CONSTRAINT `FK_stat_s_images` FOREIGN KEY (`id`) REFERENCES `s_images` (`id`) ON UPDATE CASCADE ON DELETE CASCADE;
SET FOREIGN_KEY_CHECKS=1;

4. In order to prevent deletion of records, for historical reference purposes, you can set the constraint to set NULL instead of cascade.

ALTER TABLE `favourites`
    ADD CONSTRAINT `FK_favourites_s_images` FOREIGN KEY (`image`) REFERENCES `s_images` (`id`) ON UPDATE CASCADE ON DELETE SET NULL;