Databases are a key technology in computer science that brings together fascinating theoretical topics and highly relevant practical applications. The goal of this lecture is to give an extended introduction to this interesting field, with a special focus on database query languages, their expressive power, and computational complexity. The lecture will introduce the relational data model, and then discuss theoretical and practical aspects of a variety of query languages:
- first-order logic as a query language and the relational algebra
- conjunctive queries and their unions
- navigational queries: path queries
- Datalog and its relatives
- query answering under database dependencies
The lecture focuses on core principles that apply to many types of databases alike (relational, graph-based, semantic web). Some important query answering algorithms are presented, too, but otherwise, the details of database implementation and administration are not covered.