How to add primary key to existing table in SQL Server

First, Open your SQL Server Management Studio (SSMS) and connect to your database, then navigate to the database that contains the table you want to modify.

Next, Expand the database and locate the table in Object Explorer, then right-click on the table and select Design.

In the design view, select the column(s) you want to include in the primary key by ctrl + clicking on the key icon next to the columns, then set them as the primary key.

After defining the primary key columns, save the changes by clicking the Save icon in the toolbar or by pressing Ctrl + S.

If you prefer using SQL scripts to modify your database.

ALTER TABLE TableName
ADD CONSTRAINT PK_ConstraintName PRIMARY KEY (Column1, Column2, ...);

Suppose you have a table named Customers and you want to add a primary key on the CustomerID column.

ALTER TABLE Customers
ADD CONSTRAINT PK_Customers PRIMARY KEY (CustomerID);

This script adds a primary key constraint named PK_Customers on the CustomerID column in the Customers table.