How to create a composite primary key in SQL Server 2008

sql

primary-key

database-design

sql-server

byNikita Barsukov·Jan 16, 2025

To build a composite primary key in SQL Server, identify multiple columns within the PRIMARY KEY constraint. You can use the CREATE TABLE statement for newly created tables, or the ALTER TABLE statement to modify existing ones.

Example for a new table:

CREATE TABLE my_table (
    col1 INT,             -- Hey, I'm your first column!
    col2 VARCHAR(50),     -- And I'm your second column!
    PRIMARY KEY (col1, col2)   -- Now, we both form a composite primary key!
);

Example for an existing table:

ALTER TABLE my_table
ADD PRIMARY KEY (col1, col2);

Ensure to replace col1 and col2 with the names of the columns in your table. This will enforce uniqueness across their combined values.

Constructing and validating the primary key

Success in implementing a composite key lies in understanding its properties and limitations in the context of your database. First and foremost, make sure that col1 and col2 are non-null. Allowing null values in primary keys can compromise data consistency.

The column data types and widths should be chosen according to the nature of the data they will hold and to enhance query performance. Avoid large varchar-based primary keys.

Given its impact on the physical storage of data, the CLUSTERED option should be judiciously used as the indexing strategy. The NONCLUSTERED option is more suitable for general use.

Naming conventions and constraint modifications

Benefit from the organization and ease-of-maintenance that named constraints offer. Named constraints also help in keeping track of constraints when modifications are required.

ALTER TABLE my_table
ADD CONSTRAINT pk_my_table PRIMARY KEY (col1, col2);  -- Notice the named constraint.

Keep in mind that altering a primary key requires you to drop the existing constraint before setting up a new one:

ALTER TABLE my_table
DROP CONSTRAINT pk_my_table;

ALTER TABLE my_table
ADD CONSTRAINT pk_my_table_comp PRIMARY KEY (col1, col2, col3); -- Adding an extra column to the primary key

Remember to use naming conventions that make tracking and managing constraints a breeze.

Handling the quirks of existing data

Adding a composite primary key to a populated table can present its own unique challenges. The major obstacle stems from potential duplicates in the column data. To avoid the ALTER TABLE statement flailing a 'duplicate data' error, ensure that the uniqueness feature of the composite key is retained.

Optimizing and verifying through scripts

Software tools like SQL Server Management Studio (SSMS) offer great help in creating, visualizing, and verifying the structure of your tables. The SSMS graphic interface allows even beginner SQL users to set up the primary keys.

Once created, do inspect the generated SQL scripts to acquaint yourself with the syntax. A properly planned and executed primary key can add substantial improvements to your query performance.

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