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. Each table …

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 identifier for …

There is already an primary key in your table. You can't just add primary key,otherwise will cause error.

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 corresponding …

Summary: in this tutorial, you will learn how to use the SQL PRIMARY KEY constraint to add a primary key to the table. A primary key is a column or a set of columns that uniquely identifies each row in …

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

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 professional …

Jul 23, 2025 · PRIMARY KEY in SQL is a column (or group of columns) that uniquely identifies the records in that table. A primary key must contain unique values and can not have any NULL value. …

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.

Here, the SQL command runs without errors because we have supplied the value 1 to the primary key i.e. college_id. We get this error when we supply duplicate values to the primary key, which violates …