We propose a new method for database schema validation that provides an explanation when it determines that a certain desirable property of a database schema does not hold. Explanations are required to give the designer a hint about the changes of the schema that are needed to fix the problem identified. Our method is an extension of the CQC method, which has been shown successful for testing such properties, and its contribution is twofold: Firstly, it is the first method that offers an explanation when the schema is not adequately defined. Secondly, the extension proposed here provides a significant efficiency improvement as far as the run-time performance of the method is concerned.