MySQL Cannot drop index needed in a foreign key constraint

Step 1

List foreign key ( NOTE that its different from index name )

SHOW CREATE TABLE  <Table Name>

The result will show you the foreign key name.

Format:

CONSTRAINT `FOREIGN_KEY_NAME` FOREIGN KEY (`FOREIGN_KEY_COLUMN`) REFERENCES `FOREIGN_KEY_TABLE` (`id`),

Step 2

Drop (Foreign/primary/key) Key

ALTER TABLE <Table Name> DROP FOREIGN KEY <Foreign key name>

Step 3

Drop the index.

You have to drop the foreign key. Foreign keys in MySQL automatically create an index on the table (There was a SO Question on the topic).

ALTER TABLE mytable DROP FOREIGN KEY mytable_ibfk_1 ; 

If you mean that you can do this:

CREATE TABLE mytable_d (
ID          TINYINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
Name        VARCHAR(255) NOT NULL,
UNIQUE(Name)
) ENGINE=InnoDB;


ALTER TABLE mytable
ADD COLUMN DID tinyint(5) NOT NULL,
ADD CONSTRAINT mytable_ibfk_4 
      FOREIGN KEY (DID) 
        REFERENCES mytable_d (ID) ON DELETE CASCADE;

 > OK.

But then:

ALTER TABLE mytable
DROP KEY AID ;

gives error.

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You can drop the index and create a new one in one ALTER TABLE statement:

ALTER TABLE mytable
DROP KEY AID ,
ADD UNIQUE KEY AID (AID, BID, CID, DID);