Just allow column university_id
of table user
to allow NULL
value so you can save nulls.
CREATE TABLE user
(
uid INT NOT NULL,
Name VARCHAR(30) NOT NULL,
university_ID INT NULL, -- <<== this will allow field to accept NULL
CONSTRAINT user_fk FOREIGN KEY (university_ID)
REFERENCES university(university_ID)
)
UPDATE 1
based on your comment, you should be inserting NULL
and not ''
.
insert into user (name,university_id) values ('harjeet', NULL)
UPDATE 2
$university_id = !empty($university_id) ? "'$university_id'" : "NULL";
insert into user (name,university_id) values ('harjeet', $university_id);
As a sidenote, the query is vulnerable with SQL Injection
if the value(s) of the variables came from the outside. Please take a look at the article below to learn how to prevent from it. By using PreparedStatements
you can get rid of using single quotes around values.
- How to prevent SQL injection in PHP?