Unique Constraints and NULLs in SQL Server: Navigating the Roadblocks

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Here's why:

  • Unique Constraints: These enforce that there are no duplicate values within a specific column or set of columns in a table.
  • NULL Values: While not technically data, NULL represents the absence of a known value.

However, there are ways to achieve a similar effect:

  1. Conditional Unique Index:

    • This approach involves creating a regular table that allows nulls in the desired column.
    • Then, you create a unique nonclustered index on that column with a WHERE clause that excludes null values from the uniqueness check.
    • This allows you to have multiple null entries and enforces uniqueness only for non-null values.
  2. Alternative Solutions:

    • If nulls are not a common occurrence, you can consider setting a default value for the column instead. This eliminates nulls and allows you to use a regular unique constraint.
    • Depending on your specific needs, you might explore using a CHECK constraint to define a custom validation rule for the column data.

Here are some additional points to consider:

  • By default, creating a unique constraint automatically creates a unique index.
  • Using a conditional unique index can be slightly less performant than a regular unique constraint because it needs to evaluate the WHERE clause during insert operations.

Remember, the best approach depends on your specific data model and requirements.

Conditional Unique Index:

CREATE TABLE dbo.YourTable (
  MyColumn nvarchar(50) NULL

  ON dbo.YourTable(MyColumn) 

This code:

  1. Creates a table YourTable with two columns: ID (primary key) and MyColumn (nullable).
  2. Creates a unique nonclustered index named IX_UniqueMyColumn on the MyColumn.
  3. The WHERE clause ensures only non-null values in MyColumn are considered for uniqueness.

Alternative (Default Value):

CREATE TABLE dbo.YourTable (
  MyColumn nvarchar(50) NOT NULL DEFAULT 'Unknown'

ALTER TABLE dbo.YourTable 
  ADD CONSTRAINT UC_UniqueMyColumn UNIQUE (MyColumn);

This code:

  1. Creates a table YourTable with two columns: ID (primary key) and MyColumn (not nullable) with a default value of 'Unknown'.
  2. Defines a unique constraint named UC_UniqueMyColumn on the MyColumn.

Remember to choose the approach that best suits your data and how you want to handle null values.

  1. Computed Column with Unique Constraint:

This method involves creating a computed column that combines the desired column with another unique identifier (like the primary key) but only if the original column is not null. Then, you can apply a unique constraint on the computed column.

Here's an example:

CREATE TABLE dbo.YourTable (
  MyColumn nvarchar(50) NULL

ALTER TABLE dbo.YourTable
ADD ComputedColumn AS ISNULL(MyColumn, CONCAT(ID, '~')), -- Combine ID and '~'

ALTER TABLE dbo.YourTable
ADD CONSTRAINT UC_UniqueMyColumn UNIQUE (ComputedColumn);

This code:

  1. Creates a table similar to the previous example.
  2. Defines a computed column that uses the ISNULL function.
    • If MyColumn is not null, it returns the original value.
    • If MyColumn is null, it concatenates the ID with a delimiter (e.g., '~') to create a unique identifier.
  3. Applies a unique constraint on the ComputedColumn.

Note: This method requires additional processing during data retrieval as the computed column is evaluated each time.

  1. View with NOT NULL Predicate:

This approach involves creating a view that selects only non-null rows from the original table and then creates a unique index on the view.

Here's an example:

CREATE VIEW vw_UniqueMyColumn AS
SELECT * FROM dbo.YourTable

  ON vw_UniqueMyColumn(MyColumn);

This code:

  1. Creates a view named vw_UniqueMyColumn that only selects rows where MyColumn is not null.
  2. Creates a unique clustered index on the view, enforcing uniqueness for non-null values in the original table.

Note: This method requires managing data through the view (INSERT, UPDATE, and DELETE) and might have performance implications compared to a direct table constraint.

These methods offer alternative solutions for pre-2008 SQL Server versions where conditional unique indexes weren't available. Choose the method that best suits your needs and consider the trade-offs in terms of performance and complexity.

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