title: MySQL Cannot drop index needed in a foreign key constraint
body: <p>I need to ALTER my existing database to add a column. Consequently I also want to update the UNIQUE field to encompass that new column. I'm trying to remove the current index but keep getting the error <code>MySQL Cannot drop index needed in a foreign key constraint</code></p>
  
  <pre><code>CREATE TABLE mytable_a (
  ID          TINYINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Name        VARCHAR(255) NOT NULL,
  UNIQUE(Name)
  ) ENGINE=InnoDB;
  
  CREATE TABLE mytable_b (
  ID          TINYINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Name        VARCHAR(255) NOT NULL,
  UNIQUE(Name)
  ) ENGINE=InnoDB;
  
  CREATE TABLE mytable_c (
  ID          TINYINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Name        VARCHAR(255) NOT NULL,
  UNIQUE(Name)
  ) ENGINE=InnoDB;
  
  
  CREATE TABLE `mytable` (
    `ID` int(11) NOT NULL AUTO_INCREMENT,
    `AID` tinyint(5) NOT NULL,
    `BID` tinyint(5) NOT NULL,
    `CID` tinyint(5) NOT NULL,
    PRIMARY KEY (`ID`),
    UNIQUE KEY `AID` (`AID`,`BID`,`CID`),
    KEY `BID` (`BID`),
    KEY `CID` (`CID`),
    CONSTRAINT `mytable_ibfk_1` FOREIGN KEY (`AID`) REFERENCES `mytable_a` (`ID`) ON DELETE CASCADE,
    CONSTRAINT `mytable_ibfk_2` FOREIGN KEY (`BID`) REFERENCES `mytable_b` (`ID`) ON DELETE CASCADE,
    CONSTRAINT `mytable_ibfk_3` FOREIGN KEY (`CID`) REFERENCES `mytable_c` (`ID`) ON DELETE CASCADE
  ) ENGINE=InnoDB;
  
  
  
  
  mysql&gt; ALTER TABLE mytable DROP INDEX AID;
  ERROR 1553 (HY000): Cannot drop index 'AID': needed in a foreign key constraint
  </code></pre>