Introduction
Database programs, such as Microsoft Access, are popular end-user applications that allow people to organize large quantities of data in ways that make it easy to answer questions about that data. All modern database programs are based on the relational database model which organizes logically related data into tables.
These tables, which are known as relations, are subdivided into rows and columns. Each column, or attribute, of a relation keeps up with a particular kind of data. Each row, or tuple, of a relation holds all of the data about a particular entity or event. A query may be thought of as a question posed to the database. Query operators act on existing relations and always produce new relations as output. The “select” operator is used to retrieve from a relation, particular tuples, or rows, that satisfy some test. The “project” operator is used to retrieve particular attributes, or columns, from a relation.
The Watson Lab has two main views. One is the QBE, Query by Example, which allows you to create relationships from existing data and by joining two relations. And QBRE, Query by Relational Example, which allows for projections, joins, and selections from relations.