(a) State the entity integrity rule and briefly explain its importance in database design. [2] (b) Databases use a Data Definition Language (DDL) and the Data Manipulation Language (DML). i) Differentiate between the purpose of the DDL and the purpose of the DML. [2] ii) Using MySQL, give a simple example of a DDL command and a DML command. [2] (c) Explain the concept of a subquery (or nested query) in MySQL and how it works. Give a simple, illustrative example to aid your explanation. [3]