Unable to create foreign key on mysql - mysql

Question

Hi i'm trying to create a simple MySQL database. I write the following script
CREATE TABLE office (
num_office varchar(10) NOT NULL,
name varchar(45),
address varchar(45),
numder varchar(4),
city varchar(45)
);
CREATE TABLE office_phone(
n_office varchar(10) NOT NULL,
phone varchar(15) NOT NULL,
PRIMARY KEY(n_office,phone),
FOREIGN KEY (n_office) REFERENCES office(num_office)
ON DELETE RESTRICT
ON UPDATE CASCADE
);
but Error Code 1822 is appeared.
How can i fix it?
thank you in andvance

Answer 1

Foreign keys can reference only columns that are primary keys or at least have an index on them (thanks #Uueerdo). For example:
ALTER TABLE OFFICE ADD CONSTRAINT offnum_unq UNIQUE (NUM_OFFICE);
This forces the referenced column to have unique values and also creates an index on it. Or you could also do:
CREATE INDEX offnum_idx ON OFFICE(NUM_OFFICE);
Now your second CREATE should work.

Answer 2

Reference column (Column you referenced as foreign key) should have the index added to it. You can add Index to the column "num_office" in "office" table and try creating the "office_phone" table. Refer the following queries
Create Table "office" with the index added to "num_office"
CREATE TABLE office (
num_office varchar(10) NOT NULL,
name varchar(45),
address varchar(45),
numder varchar(4),
city varchar(45),
INDEX num_office_idx (num_office)
);
OR you can add index after creating the "office" table using
ALTER TABLE office Add Index num_office_indx (num_office);
This will let you to create the table "office_phone" successfully with the reference key pointing to "num_office" column of "office" table.
CREATE TABLE office_phone(
n_office varchar(10) NOT NULL,
phone varchar(15) NOT NULL,
PRIMARY KEY(n_office,phone),
FOREIGN KEY (n_office) REFERENCES office(num_office)
ON DELETE RESTRICT
ON UPDATE CASCADE
);
Refer this page for more details