Database systems are at the core of almost all large applications and are used in all enterprises. In this lecture, we will discuss database systems from an application perspective. We start with the conceptual design of databases using ER diagrams and their translation into the relational data model.  Then, we dig into the relational algebra that enables us to operate on top of these models. We also learn the SQL querying language, i.e., how we can create, update and search data tables. The second part of this lecture discusses advanced topics such as transaction management, programming database systems. From time to time, we will leave the application perspective and learn how a database system implements user instructions, for example during query processing or for consistency assurance.