How to avoid misuse of "null value" and "empty string" in database?

To avoid misuse of "null value" and "empty string" in a database, follow these best practices:

1. Understand the Difference:

   • A null value means the data is missing, unknown, or undefined.
   • An empty string is a valid string with zero length, indicating intentional blank input.

2. Define Clear Business Rules:

   • Decide whether a field should allow NULL or enforce a default (e.g., empty string). For example, if a user’s middle name is optional, NULL may represent "not provided," while an empty string could mean "intentionally left blank."

3. Use Constraints and Defaults:

   • Set NOT NULL constraints for mandatory fields to prevent NULL values.
   • Use DEFAULT '' (empty string) for fields where blank input is valid but should be explicit.

4. Normalize Data Handling:

   • In queries, explicitly check for NULL using IS NULL or IS NOT NULL instead of relying on comparisons (e.g., WHERE column = '' won’t match NULL).
   • Example:

     -- Correct: Check for NULL explicitly
     SELECT * FROM users WHERE email IS NULL;
     
     -- Incorrect: This misses NULL values
     SELECT * FROM users WHERE email = '';
     

5. Application Layer Validation:

   • Validate input at the application level to ensure consistency. For example, if a field should not accept NULL, reject such submissions.

6. Use Database-Specific Features:

   • In TencentDB for MySQL, enable strict SQL mode (STRICT_TRANS_TABLES) to reject invalid NULL inserts.
   • For TencentDB for PostgreSQL, use CHECK constraints to enforce business rules (e.g., CHECK (email IS NOT NULL OR email = '')).

7. Document Standards:

   • Maintain a data dictionary to document whether each field allows NULL or expects empty strings, ensuring team alignment.

Example Scenario:

• A phone_number field in a customers table:
  • If NULL means "no phone provided," and an empty string means "user explicitly entered no number," define this in the schema.
  • Use NOT NULL DEFAULT '' if all phones must be recorded (even blank).

By following these practices, you can prevent ambiguity and ensure data integrity.