Abstract
This chapter introduces databases and the Structured Query Language (SQL). Most of today's data is stored in databases within tables. In order to gain knowledge and make valuable business decisions, it is necessary to extract information efficiently. This is achieved using the Structured Query Language. Good relational databases avoid repetitive data using standard normalisation approaches: for instance, by splitting raw data into multiple tables. These tables (=entities) are linked (=related) using business rules. Structured queries make use of entity relationships to obtain data. This is one of the fundamental concepts of data mining of structured data.