MySQL（InnoDB）：需要删除列，并附带外键约束和索引

Question

Here's my table:

CREATE TABLE `alums_alumphoto` (  
  `id` int(11) NOT NULL auto_increment,  
  `alum_id` int(11) NOT NULL,  
  `photo_id` int(11) default NULL,  
  `media_id` int(11) default NULL,  
  `updated` datetime NOT NULL,  
  PRIMARY KEY  (`id`),  
  KEY `alums_alumphoto_alum_id` (`alum_id`),  
  KEY `alums_alumphoto_photo_id` (`photo_id`),  
  KEY `alums_alumphoto_media_id` (`media_id`),  
  CONSTRAINT `alums_alumphoto_ibfk_1` FOREIGN KEY (`media_id`) REFERENCES `media_mediaitem` (`id`),  
  CONSTRAINT `alum_id_refs_id_706915ea` FOREIGN KEY (`alum_id`) REFERENCES `alums_alum` (`id`),  
  CONSTRAINT `photo_id_refs_id_63282119` FOREIGN KEY (`photo_id`) REFERENCES `media_mediaitem` (`id`)  
) ENGINE=InnoDB AUTO_INCREMENT=63 DEFAULT CHARSET=utf8  

I want to delete the column photo_id , which presumably will also require deleting the foreign key constraint and the index.

The problem is that I get errors when I try to drop the column:

ERROR 1025 (HY000): Error on rename of '.\dbname\#sql-670_c5c' to '.\dbname\alums_alumphoto' (errno: 150)

... when I try to drop the index (same as above), and when I try to drop the foreign key constraint:

ERROR 1091 (42000): Can't DROP 'photo_id_refs_id_63282119'; check that column/key exists)

What order should I be doing all of this in? What precise commands should I be using?

Answer 1

Precisely, try this :

First drop the Foreign Key or Constraint :

ALTER TABLE `alums_alumphoto` DROP FOREIGN KEY `photo_id_refs_id_63282119`;

The previous command removes the Foreign Key Constraint on the column. Now you can drop the column photo_id (the index is removed by MySQL on dropping the column) :

ALTER TABLE `alums_alumphoto` DROP COLUMN `photo_id`;

Aternatively, you could combine these 2 operations into one :

ALTER TABLE `alums_alumphoto` 
   DROP FOREIGN KEY `photo_id_refs_id_63282119` , 
   DROP COLUMN `photo_id`;

Answer 2

The sure thing is to make a duplicate table.

> CREATE TABLE alums_alumphoto_new LIKE alums_alumphoto;
> ALTER TABLE .... // Drop constraint
> ALTER TABLE .... // Drop KEY
> ALTER TABLE .... // Drop the column
> INSERT INTO alums_alumphoto_new (SELECT id, alum_id, photo_id, media_id, updated FROM alums_alumphoto);
> RENAME TABLE alums_alumphoto TO alums_alumphoto_old, alums_alumphoto_new TO alums_alumphoto;

If there's an error executing RENAME TABLE, some other tables might have foreign key constraints referencing this table, in which case this whole approach is stupid. :)

Answer 3

Try combining the DROP KEY and DROP FOREIGN KEY statements.

ALTER TABLE `alums_alumphoto` 
    DROP KEY KEY `alums_alumphoto_photo_id`,
    DROP FOREIGN KEY `photo_id_refs_id_63282119`;

ALTER TABLE `alums_alumphoto` 
    DROP COLUMN `photo_id`;