[英]how to pass a null value to a foreign key field?
I have 2 tables: 我有2张桌子:
university
: university
:
university_id(p.k) | university_name
and user
: 和user
:
uid | name | university_id(f.k)
How to keep university_id
NULL in user table? 如何在用户表中保持university_id
空 ?
I am writting only 1 query, my query is: 我只写了1个查询,我的查询是:
INSERT INTO user (name, university_id) VALUES ($name, $university_id);
Here $university_id
can be null from front end. 这里$university_id
从前端可以为null。
university
table will be set bydefault by me. university
表将由我设置为默认。
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. 在前端,学生将选择大学名称,根据university_id
将传递给user
表,但如果学生没有选择任何大学名称,则应将null值传递给university_id
字段的user
表。
Just allow column university_id
of table user
to allow NULL
value so you can save nulls . 只要允许列university_id
表的user
,允许NULL
值,这样可以节省空 。
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 更新1
based on your comment, you should be inserting NULL
and not ''
. 根据你的评论,你应该插入NULL
而不是''
。
insert into user (name,university_id) values ('harjeet', NULL)
UPDATE 2 更新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. 作为一个旁注,查询是脆弱的SQL Injection
,如果变量的值(一个或多个 )从外面走了进来。 Please take a look at the article below to learn how to prevent from it. 请查看下面的文章,了解如何防止它。 By using PreparedStatements
you can get rid of using single quotes around values. 通过使用PreparedStatements
您可以摆脱使用值周围的单引号。
Here suppose i have foreign key user_id
and i want to insert null value for that. 这里假设我有外键user_id
,我想为此插入空值。
Checkbox must be checked for insert null value for foreign key. 必须检查复选框是否为外键插入空值。
I was using MySQL InnoDB and even allowing NULL in the FK column and using NULL as default I was getting an error. 我使用MySQL InnoDB,甚至在FK列中允许NULL并使用NULL作为默认值我遇到了错误。 So I used the SET syntax: 所以我使用了SET语法:
INSERT INTO (table) SET value1=X...
And I just don't set the FK column. 我只是不设置FK列。
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