how to pass a null value to a foreign key field?

问题:

I have 2 tables:

university:

university_id(p.k) | university_name 

and user:

uid | name | university_id(f.k) 

How to keep university_id NULL in user table?

I am writting only 1 query, my query is:

INSERT INTO user (name, university_id) VALUES ($name, $university_id); 

Here $university_id can be null from front end.

university table will be set bydefault by me.

In the front end, student will select the university name, according to that the university_id will pass to user table, but if student is not selecting any university name then is should pass null value to the user table for university_id field.

回答1:

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?

回答2:

Here suppose i have foreign key user_id and i want to insert null value for that.

Checkbox must be checked for insert null value for foreign key.

回答3:

I was using MySQL InnoDB and even allowing NULL in the FK column and using NULL as default I was getting an error. So I used the SET syntax:

INSERT INTO (table) SET value1=X... 

And I just don't set the FK column.

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