A roll-your-own database, created in the Unix toolkit philosophy. It appears to be written in the awk language, and is very compatible with awk. It uses awk's syntax and can be combined with awk commands.
The definitive introduction is "Unix Relational Database Management: Application Development in the Unix Environment", by Rod Manis, Evan Schaeffer, and Robert Jorgensen, published by Prentice Hall. The book tells how to use rdb to create database/spreadsheets in the awk tradition, only easier. It's a good way to get into programming for novices. It's also a good way to learn DB theory and construction quite painlessly.