MySQL/POSTGRES how to force foreign key to be NULL - mysql

Question

I have the following SQLite3 database
CREATE TABLE IF NOT EXISTS users (
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
email VARCHAR(75) NOT NULL,
password VARCHAR(128) NOT NULL
);
CREATE TABLE IF NOT EXISTS pads (
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
name VARCHAR(100) NOT NULL,
user_id INTEGER NOT NULL REFERENCES users(id)
);
CREATE TABLE notes (
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
pad_id INTEGER REFERENCES pads(id),
user_id INTEGER NOT NULL REFERENCES users(id),
name VARCHAR(100) NOT NULL,
text text NOT NULL,
created_at DATETIME NOT NULL,
updated_at DATETIME NOT NULL
);
~
I have tried to migrate this sql schema to mysql and postgres, everything seems to be working fine except for one detail.
The table notes should accept pad_id as NULL, if there is no pad a note should be saved but when I try to save a note without a pad I get an error
sqlalchemy.exc.IntegrityError
IntegrityError: (psycopg2.errors.ForeignKeyViolation) insert or update
on table "note" violates foreign key constraint "note_pad_id_fkey"
DETAIL: Key (pad_id)=(0) is not present in table "pad".
But I should be able to save it as NULL, it works fine on SQLite3.
What should I change to be able to accomplish this?
Thank you.

Answer 1

Neither MySQL nor Postgres has a problem, with NULL as padod see examples
Therefore the error messahe comes from postgres, but it is because of the migration
CREATE TABLE IF NOT EXISTS users (
id INTEGER PRIMARY KEY AUTO_INCREMENT NOT NULL,
email VARCHAR(75) NOT NULL,
password VARCHAR(128) NOT NULL
);
CREATE TABLE IF NOT EXISTS pads (
id INTEGER PRIMARY KEY AUTO_INCREMENT NOT NULL,
name VARCHAR(100) NOT NULL,
user_id INTEGER NOT NULL REFERENCES users(id)
);
CREATE TABLE notes (
id INTEGER PRIMARY KEY AUTO_INCREMENT NOT NULL,
pad_id INTEGER REFERENCES pads(id),
user_id INTEGER NOT NULL REFERENCES users(id),
name VARCHAR(100) NOT NULL,
`text` text NOT NULL,
created_at DATETIME NOT NULL,
updated_at DATETIME NOT NULL
);
INSERT INTO users VALUES(NULL,'test','pass')
✓
INSERT INTO notes VALUES (NULL,NULL,1,'test','text',NOW(),NOW())
✓
db<>fiddle here
CREATE TABLE IF NOT EXISTS users (
id SERIAL PRIMARY KEY NOT NULL,
email VARCHAR(75) NOT NULL,
password VARCHAR(128) NOT NULL
);
CREATE TABLE IF NOT EXISTS pads (
id SERIAL PRIMARY KEY NOT NULL,
name VARCHAR(100) NOT NULL,
user_id INTEGER NOT NULL REFERENCES users(id)
);
CREATE TABLE notes (
id SERIAL PRIMARY KEY NOT NULL,
pad_id INTEGER REFERENCES pads(id),
user_id INTEGER NOT NULL REFERENCES users(id),
name VARCHAR(100) NOT NULL,
"text" text NOT NULL,
created_at timestamp NOT NULL,
updated_at timestamp NOT NULL
);
INSERT INTO users VALUES(DEFAULT,'test','pass')
1 rows affected
INSERT INTO notes VALUES (DEFAULT,NULL,1,'test','text',NOW(),NOW())
1 rows affected
db<>fiddle here