SQL Primary Key constraint is used to implement the data Integrity in SQL tables. When you set a SQL Server Primary key on any column, the Database engine will automatically create a Unique index on that column. In real-time, we use these key columns to identify the columns uniquely and to access the data quickly and effectively.
In general, every table has at least one column that contains unique values called the SQL Primary Key column. For example, an Employee table might have Employee ID, or Orders have an Order ID column.
Before we get into the SQL Server Primary Key example, the following are the list of points you should remember:
- A table can contain Only one Primary key in SQL Server.
- You can’t insert any duplicate content into the primary key columns.
- The SQL primary key column doesn’t allow NULL values. By default, NOT NULL will be assigned to the primary key column, even if you forgot at the time of creation.
- If you forgot to mention the index as Clustered, or Non clustered then, the Database engine will assign clustered index by default.
Create SQL Primary Key
We can create a SQL Server primary key using both the Transact SQL query and the SQL Server Management Studio.
Create SQL Primary Key using T-SQL
Let us see how to create a Primary Key using a query. Here we will create a primary key at the creation of the table. I suggest you refer to Create Table article.
USE [SQL Tutorial] GO CREATE TABLE [CustmerRecords] ( [CustomerKey] [int] NOT NULL PRIMARY KEY, [FirstName] [varchar](50) NOT NULL , [LastName] [varchar](50) NULL, [BirthDate] [date] NULL, [EmailAddress] [nvarchar](50) NULL, [Yearly Income] [money] NULL, [Profession] [nvarchar](100) NULL )
As you can see from the above code snippet, we just added the PRIMARY KEY word to the Column declaration. SSMS is intelligent enough to create a primary key for you.
Let me show you the internal code generated by the SQL, by right-clicking on the table -> script as – > create to new query window option
Create SQL Primary Key on Existing table
Let us add SQL Server Primary Key on the existing table. For this, we are going to create a new table using the following statement
CREATE TABLE [CustmerRecords] ( [CustomerKey] [int] NOT NULL, [FirstName] [varchar](50) NOT NULL , [LastName] [varchar](50) NULL, [BirthDate] [date] NULL, [EmailAddress] [nvarchar](50) NULL, [Yearly Income] [money] NULL, [Profession] [nvarchar](100) NULL )
Now let me add the SQL Primary key constraint. As you can see from the below screenshot, we are using the Alter Table Statement to alter the table content. And then, we used the ADD Constraint statement to add the primary key constraint.
USE [SQL Tutorial] GO ALTER TABLE [CustmerRecords] ADD CONSTRAINT PK_CustmerRecords_CustomerKey PRIMARY KEY CLUSTERED (CustomerKey); GO
Insert rows into SQL Server Primary Constraint Column
Let me insert a few rows to check the primary key functionality.
INSERT INTO [dbo].[CustmerRecords] VALUES (1, 'Imran', 'Khan', '10-08-1985', '[email protected]', 15900, 'Skilled Professional') ,(2, 'Doe', 'Lara', '10-08-1985', '[email protected]', 15000, 'Management') ,(3, 'Ramesh', 'Kumar', '10-08-1985', '[email protected]', 65000, 'Professional')
Let us see the inserted data.
Insert Duplicate into SQL Primary key Column
Above Insert Statement will work fine because we are inserting unique records for the customer Key. Let me insert the duplicate value into the Customer Key column.
INSERT INTO [dbo].[CustmerRecords] VALUES (1, 'Tutorial', 'Gateway', '10-08-1985', '[email protected]', 15900, 'Skilled Professional')
From the above screenshot, you can see it is throwing an error: Violation of PRIMARY KEY constraint ‘PK_CustmerRecords_CustomerKey’. Cannot insert duplicate key in object ‘dbo.CustmerRecords’. The duplicate key value is (1).
Insert Null in SQL Primary key Column
Let me insert the Null Value into the primary key column called customer.
INSERT INTO [dbo].[CustmerRecords] VALUES (NULL, 'Tutorial', 'Gateway', '10-08-1995', '[email protected]', 15900, 'Management')
It is also throwing an error.
Set SQL Primary Constraint using SSMS
To create a Primary Constraint using the SQL Server Management Studio, please go to the Object Explorer. Within the object explorer, Right-click on the table on which you want to create a primary key (here it is Custrecords) will open the context menu. Please select the Design option
Once you click the Design option, the SQL management studio will open the Custrecords table in design mode. It has 7 columns of different data types, and our job is to add a primary key to the Customer key column.
Right-click on the Customer Key column and select the Set Primary Key option from the available options.
Once you select the Set Primary Key option, SSMS will set the Primary key for you.
Modify SQL Primary Constraint
Please select the table on which your primary key holds, then go to the Indexes folder. Next, expand the Indexes folder to find the key available on that table, and Right-click on it will open the context menu. Please select the option as per your requirement
NOTE: You can’t alter the constraint using the SQL query. To modify the existing primary key, you have to delete and recreate it.
Delete SQL Primary Key using DROP Constraint
If you know the primary key constraint name, use the DROP Constraint statement along with the ALTER TABLE Statement
ALTER TABLE [CustmerRecords] DROP CONSTRAINT PK_CustmerRecords_CustomerKey
If you don’t know the SQL Primary Key constraint name, use the first SELECT Statement to get the Constraint name.
-- To find the Constraint Name SELECT name FROM sys.key_constraints WHERE type = 'PK' AND OBJECT_NAME(parent_object_id) = N'CustmerRecords'; GO -- Delete or Drop the Constraint ALTER TABLE [CustmerRecords] DROP CONSTRAINT PK_CustmerRecords_CustomerKey
Let me open the Customer records table in a designer mode to check whether we successfully removed the primary key on the Customer Key column or not.
As you can see from the above screenshot, there is no primary on the key column.