How do you create a table with a SPARSE column to optimize storage of NULL values?
HenryPk
Posted by HenryPk
Last Updated: June 17, 2024
Sparse Storage
In SQL Server, a sparse column is a column that is optimized for NULL values. When you declare a column as sparse, SQL Server uses less disk space for storing NULL values compared to regular columns. This can be very beneficial in scenarios where a large number of NULL values are expected in a table.
Steps to Create a Table with Sparse Columns
1. Create a Table: You can create a new table and define one or more of its columns as sparse. 2. Define Sparse Columns: Use the SPARSE keyword when creating or altering columns in the table.
Example
Here's a basic example to illustrate how to create a table with a sparse column: 
CREATE TABLE Employee
(
    EmployeeID INT PRIMARY KEY,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    Email NVARCHAR(100) SPARSE NULL,          -- Sparse column
    PhoneNumber NVARCHAR(15) SPARSE NULL,     -- Another sparse column
    Address NVARCHAR(250) NULL                  -- Regular column
);
Explanation:
- In the example above, Email and PhoneNumber are designated as sparse columns. This means that if these columns contain NULL values, SQL Server will use less storage space for those entries compared to a regular column. - The Address column, while it allows NULL values, is a standard column and will consume storage space for NULL values.
Considerations
- Performance: Sparse columns can improve storage efficiency but may result in slightly worse performance for non-NULL lookups because of the overhead involved. - Limitations: Sparse columns have some limitations: - You cannot use sparse columns as part of a PRIMARY KEY or UNIQUE constraint. - You should avoid using them in certain index types (e.g., filtered indexes) as they are generally designed for either sparse or dense data.
Modifying Existing Columns to Sparse
If you already have a table and want to modify an existing column to be sparse, you can do so with the ALTER TABLE command: 
ALTER TABLE Employee
ALTER COLUMN Email NVARCHAR(100) SPARSE NULL;
This will change the Email column to be a sparse column in the existing Employee table.
Conclusion
Using sparse columns is a practical method for optimizing storage for tables in SQL Server that contain a significant number of NULL values. Remember to weigh the advantages of space savings against potential performance impacts when designing your database schema.
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