Insert empty string value in a foreign key field - sql

Question

I have an error when trying to insert values with a '' in the foreign key field.
I use PostgreSQL v10.
My sql code:
CREATE TABLE meas_name (
nopol varchar(2) NOT NULL,
cchim varchar(10) NOT NULL,
ncon varchar(30) NOT NULL,
PRIMARY KEY (nopol)
);
CREATE TABLE main_device (
ntypapp smallint NOT NULL,
no_main smallint NOT NULL,
lib_main varchar(25),
nopol varchar(2),
no_chrono smallint,
PRIMARY KEY (no_main,ntypapp)
);
ALTER TABLE main_device ADD CONSTRAINT fk_maindevice_measname FOREIGN KEY (nopol) REFERENCES meas_name(nopol) ON DELETE NO ACTION NOT DEFERRABLE INITIALLY IMMEDIATE;
When i insert something in measure with a known deviceno I have this error:
insert into main_device values
(1, 1, 'TST LASER','',1578)
ERROR: insert or update on table « main_device » violates foreign key constraint « fk_maindevice_measname »
DETAIL: Key (nopol) = () is not present in table « meas_name ».
I have tested it on: https://www.db-fiddle.com/f/fBnNSYWU8qZwFbmEoU5dce/1
There was no issue on Oracle database, but I don't know much about Postgres, Is there any limitation? How can I manage this?

Answer 1

An empty string is not NULL. Use NULL:
insert into main_device (ntypapp, no_main, lib_main, nopol, no_chrono)
values (1, 1, 'TST LASER', NULL, 1578);
A NULL value does not need to match the foreign key constraint (in this context, NULL means "no value"). However, an empty string is a valid value and the database expects a matching row in the referenced table.
I also added the explicit column names because that is a best-practice.