SQL PRIMARY KEY Constraint The PRIMARY KEY constraint is used to uniquely identify each record in a table. Primary keys must contain unique values, and cannot contain NULL values. …

The table already has an existing compound-key [PersonId, Pname] as the primary key. Now I want to modify that existing compound-key to have one more field [PMid].

May 13, 2025 · Learn to add a primary key to existing SQL tables with ALTER TABLE in SQL Server, MySQL, PostgreSQL, and Oracle.

Aug 5, 2019 · This article demonstrates how to add a primary key to an existing table in SQL Server using Transact-SQL. A primary key is a column that has been configured as the unique …

Nov 18, 2025 · 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 …

Here you will learn what is a primary key and how to create it in a new or existing table in the SQL Server database.

Jul 23, 2025 · A primary key can be defined on a single column (simple primary key) or multiple columns (composite primary key). Creating a primary key automatically creates a unique index …

We can create this Primary Key constraint using the Transact query and the Management Studio. Let us see how to create a Primary Key using a transact query and create a table statement.

Jun 28, 2024 · This tutorial covers SQL Primary Key definitions, rules, How to add a primary key to an existing table in SQL, interesting facts, and more with detailed examples.

Dec 12, 2025 · This tutorial guides you through the complete process of adding a primary key to an existing table using T-SQL and SQL Server Management Studio (SSMS), tailored for a …