我有两张桌子:
university:
university_id(p.k) | university_name
和user:
uid | name | university_id(f.k)
如何将university_id-NULL保存在用户表中？
我只写了1个查询，我的查询是:
INSERT INTO user (name, university_id) VALUES ($name, $university_id);
这里$university_id可以从前端为空.
university桌将由我默认设置.
在前端，学生将 Select 大学名称，根据university_id将传递到user表，但如果学生没有 Select 任何大学名称，则应将空值传递到user表的university_id字段.
只需允许表user的第university_id列允许NULL值，就可以保存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
根据你的 comments ，你应该插入NULL而不是''.
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);
另外，如果变量的值(s)来自外部，则查询容易受到SQL Injection的攻击.请看下面的文章，了解如何预防它.通过使用PreparedStatements，您可以避免使用单引号将值引起来.