how to make a composite foreign key (not composite primary key) as unique in mysql

Question

I have a table and in that table i have a primary key, and a composite foreign key(combination of 2 foreign keys). I want to know how do I create a composite foreign key as unique which means that I don't want that composite foreign key duplicate again in the table?

this is the query for it

create table official_baby_toys(
official_baby_toys_id int,
baby_id int,
toy_id int,
toy_size_id,
primary key(official_baby_toys_id)
foreign key(baby_id,toy_id) references baby_toy(baby_id,toy_id),
foreign key(toy_size_id) references toy_size(toy_size_id),
)

Answer 1

First create two single foreign keys, then create the UNIQUE KEY with both fields:

create table official_baby_toys(
official_baby_toys_id int,
baby_id int,
toy_id int,
toy_size_id,
primary key(official_baby_toys_id)
foreign key(baby_id) references baby_toy(baby_id),
foreign key(toy_id) references baby_toy(toy_id),
foreign key(toy_size_id) references toy_size(toy_size_id),
unique key(baby_id, toy_id)
)

Also I don't know your DB schema, but I feel like baby_id should actually reference to baby(baby_id) rather than baby_toy(baby_id) .

Edit: You can also create a composite foreign key, but you need to create index (or a primary key, which will also create an index) for these two fields first:

CREATE TABLE baby_toy (
  # other field defs
  baby_id int,
  toy_id int,
  primary key(baby_id, toy_id)
  # OR index(baby_id, toy_id)
);

Then in the official_baby_toys you should be able to create composite foreign key.