Question

I just wanna ask coz I created a table with a column with two primary keys as shown below:

CREATE TABLE Parent(
Mother varchar(50) NOT NULL,
Father varchar(50) NOT NULL,
PRIMARY KEY(Mother, Father),
Organization varchar(50) NOT NULL
);

Then I am trying to create another table which will have a foreign key to the table shown above:

CREATE TABLE Child(
Name varchar(50) NOT NULL PRIMARY KEY,
Child_Mother varchar(50) REFERENCES Parent(Mother),
Child_Father varchar(50) REFERENCES Parent(Father),
Sibling varchar(50) NOT NULL
);

So I basically am trying to references two columns on the Child Table to a single column from the Parent table with two primary keys. Is this even possible? Thank you very much! 🙂

*Sample Tables only. But the actual are similar to this.

Answer 1

Try this one –

CREATE TABLE Child(
    Name varchar(50) NOT NULL PRIMARY KEY,
    Child_Mother varchar(50),
    Child_Father varchar(50),
    Sibling varchar(50) NOT NULL,
    Foreign Key (Child_Mother, Child_Father) references Parent(Mother, Father)
);

Since you are creating a primary key which consists of two columns, you need to refer to them together.

What paxdiablo said is true. You are creating a composite primary key which consists of two columns. I would suggest you to use a Surrogate Primary Key.

Edit

CREATE TABLE Child(
    id bigint NOT NULL auto_increment,  // an "id" column
    Name varchar(50) NOT NULL unique,
    age int not null,
    primary key (id)    // "id" column is being declared as surrogate primary key
);

Second Edit

create table Subject(

    id int not null,
    Subject_ID varchar(50) NOT NULL,
    Subject_Description varchar(50) NOT NULL,
    PRIMARY KEY(id),
    Subject_SID int references Student(Student_ID),
    Unit varchar(50) NOT NULL
);

ALTER TABLE Subject ADD CONSTRAINT subject_unique UNIQUE (Subject_id, Subject_Description);

create table Schedule(
    Day_MTWTHF varchar(50) NOT NULL,
    TIME_HH varchar(50) NOT NULL,
    subject_id int not null,                   -- my newly added column
    Schedule_SID int references Student(Student_ID),
    foreign key(subject_id) references Subject(id)
);