SQL PRIMARY KEY Constraint. The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

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 a given table.

In this table I have 5 fields: 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]. How can I write an ALTER statement to do this? Are you sure? this means you are allowed to have duplicate personId in your table.

Feb 4, 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 unique clustered index.

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.

In this tutorial, you will learn how to use the SQL PRIMARY KEY constraint to add a primary key to the table.

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax: PRIMARY KEY (PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).

In the below code I am adding primary key as a constraint. you can add another constraint also instead of primary key like foreign key, default etc. k. friend command: sql> alter table tablename add primary key (col_name); ex: alter table pk_Product_Detils add primary key (Product_Detail_ID);

Dec 12, 2024 · There are two syntaxes to create/add primary key to a table: SQL primary key syntax with CREATE TABLE statement is: CONSTRAINT pk_constraint_name PRIMARY KEY (column1, column2, …) SQL primary key syntax with ALTER TABLE statement is. Let’s look at some examples of the PRIMARY KEY Constraint in SQL, and understand it’s working.

This SQL tutorial explains how to create and drop a primary key in SQL with syntax and examples. What is a primary key in SQL? In SQL, a primary key is a single field or combination of fields that uniquely defines a record. None of the fields that are part of the primary key can contain a NULL value. A table can have only one primary key.