Enumerations are a fundamental concept in relational databases, but they are often misunderstood and poorly implemented. In this article, we will discuss the correct and incorrect ways to handle enumerations in SQL Server.

The Concept

Let’s consider a table called “Person” in our database. This table has properties such as Firstname, Lastname, Birthday, and MaritalStatus. MaritalStatus is an enumeration, which in C# would be implemented as an enumeration with values like Single, InRelationship, Married, and Divorced.

However, SQL Server does not have built-in support for enumerations. So, we need to find a way to represent the enumeration values in the database.

The Wrong Way

One common but incorrect approach is to use a varchar field to store the enumeration values. For example:

CREATE TABLE [dbo].[Person] (
    [Firstname] NVARCHAR(100),
    [Lastname] NVARCHAR(100),
    [Birthday] DATETIME,
    [MaritalStatus] NVARCHAR(10)
)

This approach has several downsides. Firstly, it can lead to database fragmentation if the length of the field is not sufficient to accommodate new values. Additionally, it does not enforce any constraints on the values, allowing for the possibility of entering invalid or misspelled values.

The Correct Way

A better approach is to create a separate table to represent the enumeration values. Here’s how it can be done:

CREATE TABLE [CodeNamespace] (
    [Id] INT IDENTITY(1, 1),
    [Name] NVARCHAR(100) NOT NULL,
    CONSTRAINT [PK_CodeNamespace] PRIMARY KEY ([Id]),
    CONSTRAINT [IXQ_CodeNamespace_Name] UNIQUE NONCLUSTERED ([Name])
)

INSERT INTO [CodeNamespace] SELECT 'MaritalStatus'

CREATE TABLE [CodeValue] (
    [CodeNamespaceId] INT NOT NULL,
    [Value] INT NOT NULL,
    [Description] NVARCHAR(100) NOT NULL,
    [OrderBy] INT,
    CONSTRAINT [PK_CodeValue] PRIMARY KEY CLUSTERED ([CodeNamespaceId], [Value]),
    CONSTRAINT [FK_CodeValue_CodeNamespace] FOREIGN KEY ([CodeNamespaceId]) REFERENCES [CodeNamespace] ([Id])
)

-- 1 is the 'MaritalStatus' namespace
INSERT INTO [CodeValue] SELECT 1, 1, 'Single', 1
INSERT INTO [CodeValue] SELECT 1, 2, 'In relationship', 2
INSERT INTO [CodeValue] SELECT 1, 3, 'Married', 3
INSERT INTO [CodeValue] SELECT 1, 4, 'Divorced', 4

In this approach, we create a “CodeNamespace” table to store the name of the enumeration. We then create a “CodeValue” table to hold the actual values, with a foreign key reference to the “CodeNamespace” table. This allows us to group the values under different namespaces.

Now, we can update the “Person” table to use the enumeration values:

CREATE TABLE [dbo].[Person] (
    [Firstname] NVARCHAR(100),
    [Lastname] NVARCHAR(100),
    [Birthday] DATETIME,
    [MaritalStatus] INT
)

INSERT INTO [Person] SELECT 'John', 'Doe', '1990-01-01', 3

One limitation of this approach is that the “MaritalStatus” column does not have a database-enforced relationship to the “CodeValue” table. However, this can be handled in the application layer by providing a dropdown list or combobox with the valid values. Additionally, the entered value can be validated in the data access layer.

Conclusion

When dealing with enumerations in SQL Server, it is important to follow best practices to ensure data integrity and maintainability. By creating separate tables for the enumeration values and using foreign key references, we can achieve a more robust and scalable solution.

Do you have a similar design setup in your organization? Let us know your thoughts and experiences in the comments below!