SQL Primary Key

SQL Primary Key constraint is used to implement the data Integrity in SQL tables. In general, every table has at least one column that contains unique values. For example, Employee table might have Employee ID, or Orders have an Order ID column.

When you set a 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. Before we get into the example, following are the list of points you should remember:

• A table can contain Only one Primary key.
• You can’t insert any duplicate content into the primary key columns.
• Primary key column doesn’t allow NULL values. By default NOT NULL will be assigned to 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, 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.

Set SQL Primary Key using SSMS

In this example we will show you, How to create a Primary Key in SQL server using the SQL server management Studio. To do so, Please go to the Object Explorer. Within the object explorer, Expand the Database folder in which the table had. Please select the table on which you want to create a primary key (here it is Custrecords), and Right click on it will open the context menu. Please select Design option as shown below

Once you click the Design option, SQL management studio will open the corresponding table in design mode. From the below screenshot you can observe that, We had 7 columns of different data types, and our task is to add the primary key to Customer key column.

Right click on the Customer Key column will open the context menu. Please 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.

Create SQL Primary Key using T-SQL

In this example we will show you, How to create a Primary Key in SQL server using the Transact SQL statement. here we will create a primary key at the creation of table. I Suggest you to refer Create Table article to understand the Create Table statement.

SQL CODE

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.

OUTPUT

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

In this example we will show you, How to add Primary Key on existing table. For this, we are going to create a new table using the following statement

SQL CODE

Now let me add the 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 used the ADD Constraint statement to add the primary key constraint.

SQL CODE

OUTPUT

Insert rows into SQL Server Primary Key Column

Let me insert few rows to check the primary key functionality.

SQL CODE

OUTPUT

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.

SQL CODE

OUTPUT

From the above screenshot you can see that, 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 customer key column.

SQL CODE

OUTPUT

It is also throwing an error.

Modify the SQL Primary Key

Within the object explorer, Expand the Database folder in which the table had. Please select the table on which your primary key holds, then go to the Indexes folder. Please 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. In order to alter the existing primary key, you have to delete and recreate it.

Delete SQL Primary Key using T-SQL

If you know the primary key constraint name then use the DROP Constraint statement along with the ALTER TABLE Statement

SQL CODE

If you don’t know the constraint name then use the first SELECT Statement to get the Constraint name.

SQL CODE

OUTPUT

OUTPUT 2

Let me open the Custmer records table in designer mode to check whether we successfully removed the primary key on Customer Key column or not.

As you can see from above screenshot, there is no primary on the key column.

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