MySQL NULL Values
In MySQL, NULL is a special marker used to indicate that a data value does not exist in the database.
Basic Understanding:
Example: Basic Concept of NULL
NULL in MySQL represents missing or unknown data. It is different from an empty string or zero.
Using NULL in Columns:
Example: Defining a Column to Allow NULL
CREATE TABLE employees (
employee_id INT AUTO_INCREMENT PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
birth_date DATE,
hire_date DATE,
salary DECIMAL(10, 2),
department_id INT,
manager_id INT,
termination_date DATE
);
In this example, the termination_date
column is defined to allow NULL values, indicating that not all employees may have a termination date recorded.
Querying NULL Values:
Example: Querying Rows with NULL Values
SELECT * FROM employees
WHERE termination_date IS NULL;
This SQL query selects all columns from the employees
table where the termination_date
is NULL, retrieving employees who have not been terminated.
Inserting NULL Values:
Example: Inserting a Row with NULL Value
INSERT INTO employees (first_name, last_name, birth_date, hire_date, salary, department_id, manager_id)
VALUES ('Alice', 'Smith', '1990-05-15', '2015-07-01', 60000.00, 101, NULL);
In this SQL statement, the manager_id
is assigned a NULL value, indicating that Alice Smith does not report to any specific manager.
Conclusion
Understanding NULL values in MySQL is crucial for database design and querying, allowing for flexible data handling when information may be missing or unknown.