Unique Constraints and NULLs in SQL Server: Navigating the Roadblocks
- 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:
-
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.
-
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.
CREATE TABLE dbo.YourTable (
ID int PRIMARY KEY,
MyColumn nvarchar(50) NULL
);
CREATE UNIQUE NONCLUSTERED INDEX IX_UniqueMyColumn
ON dbo.YourTable(MyColumn)
WHERE MyColumn IS NOT NULL;
This code:
- Creates a table
YourTable
with two columns:ID
(primary key) andMyColumn
(nullable). - Creates a unique nonclustered index named
IX_UniqueMyColumn
on theMyColumn
. - The
WHERE
clause ensures only non-null values inMyColumn
are considered for uniqueness.
Alternative (Default Value):
CREATE TABLE dbo.YourTable (
ID int PRIMARY KEY,
MyColumn nvarchar(50) NOT NULL DEFAULT 'Unknown'
);
ALTER TABLE dbo.YourTable
ADD CONSTRAINT UC_UniqueMyColumn UNIQUE (MyColumn);
- Creates a table
YourTable
with two columns:ID
(primary key) andMyColumn
(not nullable) with a default value of 'Unknown'.
- 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 (
ID int PRIMARY KEY,
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);
- Creates a table similar to the previous example.
- 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 theID
with a delimiter (e.g., '~') to create a unique identifier.
- If
- Applies a unique constraint on the
ComputedColumn
.
Note: This method requires additional processing during data retrieval as the computed column is evaluated each time.
- 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.
CREATE VIEW vw_UniqueMyColumn AS
SELECT * FROM dbo.YourTable
WHERE MyColumn IS NOT NULL;
CREATE UNIQUE CLUSTERED INDEX IX_UniqueMyColumn_View
ON vw_UniqueMyColumn(MyColumn);
- Creates a view named
vw_UniqueMyColumn
that only selects rows whereMyColumn
is not null. - Creates a unique clustered index on the view, enforcing uniqueness for non-null values in the original table.
sql-server t-sql