Which database constraint best enforces a valid postal code format for a country within an Address table?

Which database constraint best enforces a valid postal code format for a country within an Address table?

• A. A universal regex for postal_code across all countries.
• B. A simple length check on postal_code.
• C. A country-specific check constraint (regex) on postal_code, potentially driven by country_code.
• D. Validation occurs only at application layer.

Answer: (C)

Enforcing postal code formats at the database level uses a check constraint to validate the value against a pattern. Because postal codes vary so much by country, the best approach is to apply a country-specific pattern for postal_code, guided by country_code. This lets the constraint test the code against the correct regular expression for that country, so valid formats (like numeric or alphanumeric patterns, with or without spaces or hyphens) pass, while invalid formats fail. A universal regex across all countries is too broad or too narrow and can’t capture each country’s exact rules. A simple length check misses important detail about allowed characters and separators, leading to false positives or negatives. Relying only on the application layer invites bypasses and inconsistent validation across different apps. A country-specific, regex-based check constraint provides precise, scalable validation right where the data is stored, and can be extended as new countries are added or formats change.