Getting to Know Relational Databases - Beginning C# 2008 Databases: From Novice to Professional - page 33

Figure 3-2. A many-to-many relationship
n Note Though relationships among tables are extremely important, the term relational database has
nothing to do with them. Relational databases are (to varying extents) based on the relational model of data
invented by Dr. Edgar F. Codd at IBM in the 1970s. Codd based his model on the mathematical (set-
theoretic) concept of a relation . Relations are sets of tuples that can be manipulated with a well-defined
and well-behaved set of mathematical operations—in fact, two sets: relational algebra and relational cal-
culus . You don't have to know or understand the mathematics to work with relational databases, but if you
hear it said that a database is relational because it “relates data,” you'll know that whoever said it doesn't
understand relational databases.
Understanding Keys
The key, the whole key, and nothing but the key, so help me Codd.
Relationships are represented by data in tables. To establish a relationship between two
tables, you need to have data in one table that enables you to find related rows in another