MySQL - Error Code 1215, cannot add foreign key constraint

Question

i got these two succesfull queries:

create table Donors (
    donor_id int not null auto_increment primary key,
    gender varchar(1) not null,
    date_of_b         


        

Answer 1

To define a foreign key, the referenced parent field must have an index defined on it.

As per documentation on foreign key constraints:

REFERENCES tbl_name (index_col_name,...)

Define an INDEX on condition_code in parent table Donors_Medical_Condition and it should be working.

create table Donors_Medical_Condition (
    donor_id int not null,
    condition_code int not null,
    seriousness text,

    KEY ( condition_code ), -- <---- this is newly added index key

    primary key(donor_id, condition_code),
    foreign key(donor_id) references Donors(donor_id)    );

But it seems you defined your tables order and references wrongly. You should have defined foreign key in Donors_Medical_Condition table but not in Donors_Medical_Conditions table. The latter seems to be a parent.

Modify your script accordingly.

They should be written as:

-- create parent table first ( general practice )
create table Medical_Conditions (
    condition_code int not null,
    condition_name varchar(50) not null,
    condition_description text,
    other_details text,
    primary key(condition_code)
);

-- child table of Medical_Conditions 
create table Donors_Medical_Condition (
    donor_id int not null,
    condition_code int not null,
    seriousness text,
    primary key(donor_id, condition_code),
    foreign key(donor_id) references Donors(donor_id),
    foreign key(condition_code) 
        references Donors_Medical_Condition(condition_code)
);

Refer to:

• MySQL Using FOREIGN KEY Constraints

[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]

reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION