如果您有以下要求：

• a用户可以是教授 - 不是


• a教授总是一个用户

那么你是正确的，它是一个 1 :: 0..1 关系。在SQL中，可以这样实现：

  CREATE TABLE用户
（UserId INT NOT NULL 
 ，... 
，PRIMARY KEY（UserId）
）; 

 CREATE TABLE Professors 
（UserId INT NOT NULL 
，... 
，PRIMARY KEY（UserId）
，FOREIGN KEY（UserId）
参考用户（UserId）
）; 
 

根据您所描述的内容，您可能已经以相反的顺序定义了外键约束。

I have tables:

Users{UserId ...}
Professors{UserId ...}

I set UserId to be PK in both tables and made 1:1 relationship.

But if I try to insert new user it doesn't work as it requires insert in the Professor table too.

I want to make that one user can have only 1 record in Professor table but I also want to make it that it doesn't have to exist in Professor table (I don't want to make all users professors :) ).

How to set 1 to (0...1) relationship in SQL Server Management Studio?

I know that set enforce key constraints to NO is not a solution :)

If you have these requirements:

• a User can be Professor - or not
• a Professor is always a User

then you are correct that it's a 1 :: 0..1 relationship. In SQL, it can be implemented like this:

CREATE TABLE Users
  ( UserId INT NOT NULL
  , ...
  , PRIMARY KEY (UserId)
  ) ;

CREATE TABLE Professors
  ( UserId INT NOT NULL
  , ...
  , PRIMARY KEY (UserId)
  , FOREIGN KEY (UserId)
      REFERENCES Users (UserId)
  ) ;

From what you describe, you probably have defined the foreign key constraint in reverse order.