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Create primary keys

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Applies to: SQL Server 2016 (13.x) and later versions Azure SQL Database Azure SQL Managed Instance SQL database in Microsoft Fabric

You can define a primary key in the SQL Server Database Engine by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index. However, your primary key can be specified as a nonclustered index instead.

The code samples in this article use the AdventureWorks2022 or AdventureWorksDW2022 sample database, which you can download from the Microsoft SQL Server Samples and Community Projects home page.

Limitations

A table can contain only one PRIMARY KEY constraint.

All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL. If nullability isn't specified, all columns participating in a PRIMARY KEY constraint have their nullability set to NOT NULL.

Permissions

Creating a new table with a primary key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.

Creating a primary key in an existing table requires ALTER permission on the table.

Use SQL Server Management Studio

  1. In Object Explorer, right-click the table to which you want to add a unique constraint, and select Design.
  2. In Table Designer, select the row selector for the database column you want to define as the primary key. If you want to select multiple columns, hold down the CTRL key while you select the row selectors for the other columns.
  3. Right-click the row selector for the column and select Set Primary Key.

Caution

If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.

A primary key column is identified by a primary key symbol in its row selector.

If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique.

If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table. However, you can change the order of columns after the primary key is created. For more information, see Modify Primary Keys.

Use Transact-SQL

Create a primary key in an existing table

The following example creates a primary key on the column TransactionID in the AdventureWorks2022 database.

ALTER TABLE [Production].[TransactionHistoryArchive]
   ADD CONSTRAINT PK_TransactionHistoryArchive_TransactionID PRIMARY KEY CLUSTERED (TransactionID);

Create a primary key in a new table

The following example creates a table and defines a primary key on the column TransactionID in the AdventureWorks2022 database.

CREATE TABLE [Production].[TransactionHistoryArchive1] (
    TransactionID INT IDENTITY(1, 1) NOT NULL,
    CONSTRAINT PK_TransactionHistoryArchive1_TransactionID PRIMARY KEY CLUSTERED (TransactionID)
);

Create a nonclustered primary key with separate clustered index in a new table

The following example creates a table in the AdventureWorks2022 database, with a nonclustered primary key on the column CustomerID. Then, it adds a clustered index on TransactionID.

  1. Create a table to add the clustered index.

    CREATE TABLE [Production].[TransactionHistoryArchive1] (
    CustomerID UNIQUEIDENTIFIER DEFAULT NEWSEQUENTIALID(),
    TransactionID INT IDENTITY(1, 1) NOT NULL,
    CONSTRAINT PK_TransactionHistoryArchive1_CustomerID PRIMARY KEY NONCLUSTERED (CustomerID)
);

  2. Now add the clustered index.

    CREATE CLUSTERED INDEX CIX_TransactionID
ON [Production].[TransactionHistoryArchive1] (TransactionID);

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