In a database, a null value represents the absence of a value or an unknown value in a column of a table. It is not the same as zero (0), an empty string (''), or a space (' '). Instead, it indicates that the data is missing, not applicable, or simply not provided for a particular record.
For example, consider a database table named Employees with the following columns: EmployeeID, Name, Department, and Salary. If an employee has not yet been assigned to a department, the Department field for that employee might be set to null, meaning the department information is currently unavailable or not applicable.
| EmployeeID | Name | Department | Salary |
|---|---|---|---|
| 1 | Alice | HR | 5000 |
| 2 | Bob | NULL | 4500 |
| 3 | Charlie | IT | 6000 |
In this case, Bob's department is unknown or not assigned, so it is stored as null.
When querying the database, you need to handle null values carefully because comparisons with null using operators like = or != do not work as expected. For instance, WHERE Department = NULL will not return any rows. Instead, you should use IS NULL or IS NOT NULL. For the above example, to find employees without a department, you would write:
SELECT * FROM Employees WHERE Department IS NULL;
In cloud-based database services, such as those offered by Tencent Cloud, you can manage and query null values just like in traditional databases. Tencent Cloud provides managed database solutions like TencentDB for MySQL, TencentDB for PostgreSQL, and TencentDB for SQL Server, all of which fully support standard SQL null handling. These services allow you to store, query, and manipulate null values efficiently while ensuring high availability and scalability.