Change or drop a foreign key

from the Artful Common Queries page


To change a foreign key, first drop it, then declare the new, revised foreign key. The syntax for declaring a foreign key is ...
[CONSTRAINT [constraint_name]] 
FOREIGN KEY [key_name] (keycol_name,...) reference_definition
and the syntax for dropping one is ...
DROP FOREIGN KEY constraint_name
Notice that you can omit the CONSTRAINT when you declare a foreign key, but the only way to DROP a foreign key is to reference it by the constraint_name which you probably never specified! There should be a circle of hell reserved for designers who build inconsistencies like this into their tools. The only way round this one is to run SHOW CREATE TABLE to find out what the foreign key's constraint_name is, so you can write the DROP statement. Here is a wee test case:
drop table if exists a,b;
create table a(i int primary key)engine=innodb;
create table b(i int,foreign key(i) references a(i)) engine=innodb;
show create table\G

CREATE TABLE `b` (
  `i` int(11) DEFAULT NULL,
  KEY `i` (`i`),
  CONSTRAINT `b_ibfk_1` FOREIGN KEY (`i`) REFERENCES `a` (`i`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

-- drop and recreate the FK:
alter table b drop foreign key b_ibfk_1;
alter table b add foreign key(i) references a(i) on update cascade;
show create table b\G

Create Table: CREATE TABLE `b` (
  `i` int(11) DEFAULT NULL,
  KEY `i` (`i`),
  CONSTRAINT `b_ibfk_1` FOREIGN KEY (`i`) REFERENCES `a` (`i`) ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1

drop table a,b;

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