复合主键+外键

Question

I have a table which contains a list of Surveys (PK is ID)

CREATE TABLE [dbo].[SurveyMaster](
    [ID] [nvarchar](4) NOT NULL,
    [Title] [nvarchar](200) NULL,
    [IsActive] [bit] NOT NULL,

And a table which contains a list of Variable Mappings.

CREATE TABLE [dbo].[VariableMappings](
    [ParentSurvey_ID] [nvarchar](4) NOT NULL,
    [ReportingMonth] [nvarchar](6) NOT NULL,
    [VariableName] [nvarchar](400) NOT NULL,
    [Value] [int] NOT NULL

My hope was to create a primary key for VariableMappings on ParentSurvey_ID, ReportingMonth and Variable Name to ensure a unique record.

At the same time though, I'd like a foreign key relationship between VariableMappings.ParentSurvey_ID and SurveyMaster.ID to ensure VariableMappings only contains relevant SurveyID's.

I've tried a couple of approaches in SQL Server but I don't believe I can create the FK due to the composite key being made up of 3 columns.

How can I go about achieving this?

Answer 1

Yes, you can:

CREATE TABLE [dbo].[VariableMappings](
    [ParentSurvey_ID] [nvarchar](4) NOT NULL,
    [ReportingMonth] [nvarchar](6) NOT NULL,
    [VariableName] [nvarchar](400) NOT NULL,
    [Value] [int] NOT NULL,
  PRIMARY KEY (ParentSurvey_ID, ReportingMonth, VariableName),
  FOREIGN KEY (ParentSurvey_ID)
    REFERENCES dbo.SurveyMaster (ID)
) ;

---

Mostly irrelevant to your problem, but having a PRIMARY KEY that is so wide (410 nvarchars) is not the best idea.

Answer 2

Your primary key definition and foreign key definitions are orthogonal. You can declare your composite primary key and your single column foreign key without issue. I would suggest to you, though, that a surrogate primary key on VariableMappings is better choice vs. the composite key - specifically for foreign keys pointing at VariableMappings .

Answer 3

There is one primary key that does matter and it is that of SurveyMaster . As long as ID is the primary key there, you can reference it from any number of other tables, regardless of those tables' primary keys.

A foreign key requires that the referenced combination of columns is unique, and the uniqueness must be guaranteed, for example by a primary key constraint.