SQL Primary Key Tutorial

Example Table

We will create a table named employees to demonstrate the SQL Primary Key constraint:

        CREATE TABLE employees (
            employee_id INT PRIMARY KEY,
            first_name VARCHAR(50),
            last_name VARCHAR(50),
            department VARCHAR(50)
        );
        

Creating the Table

To create the table, use the following SQL command:

        CREATE TABLE employees (
            employee_id INT PRIMARY KEY,
            first_name VARCHAR(50),
            last_name VARCHAR(50),
            department VARCHAR(50)
        );
        

Result:

Command	CREATE TABLE employees (employee_id INT PRIMARY KEY, first_name VARCHAR(50), last_name VARCHAR(50), department VARCHAR(50));
Result	A table named employees is created with employee_id as the primary key.

Inserting Data

To insert data into the table, use the following SQL command:

        INSERT INTO employees (employee_id, first_name, last_name, department) 
        VALUES (1, 'John', 'Doe', 'HR');
        

Result:

Command	INSERT INTO employees (employee_id, first_name, last_name, department) VALUES (1, 'John', 'Doe', 'HR');
Result	Data is inserted into the employees table.

Attempting to Insert Duplicate Primary Key

If you try to insert a duplicate value into the primary key column, you will get an error:

        INSERT INTO employees (employee_id, first_name, last_name, department) 
        VALUES (1, 'Jane', 'Smith', 'Finance');
        

Result:

Command	INSERT INTO employees (employee_id, first_name, last_name, department) VALUES (1, 'Jane', 'Smith', 'Finance');
Result	Error: Duplicate entry '1' for key 'PRIMARY'.

Important Considerations

When using the Primary Key constraint, consider the following:

• Primary keys must contain unique values.
• A table can have only one primary key, which may consist of single or multiple columns.
• Primary keys ensure that each record in the table is unique and can be identified uniquely.