explain the difference between DDL (data definition language) DCL (data control language) and DML (data manipulation language) ? talking relationship between tables in database what do these concepts mean: one-to-many relationships one-to-one relationships many-to-many relationships purpose of a Primary and Foreign key what are there purposes in a database? what is referential integrity? For example if you have two tables: Invoices and Vendors and you attempt to enter a vendor_id in the Invoices table that does not exist in the Vendors table what does referential integrity enforce? come to database design why is Normalization so important? In fact, what is Data Normalization?