What data validation can be used in a database?

A database is only as useful as the data it contains. Validation helps prevent invalid or inconsistent data from getting stored. At the most elementary level, it could be as simple are requiring a given element to only contain numerical data. More complex validation rules might entail a list of valid values, cross-field edits (if field A contains "xyz", then field B cannot contain "abc") and various more complex rules known as constraints (such as foreign key and NOT NULL rules.)