What’s a Relational Database Management System? - dummies

What’s a Relational Database Management System?

By Thomas C. Hammergren

Forget all about the mathematical foundations of the relational model, the principles of normalization, and other highly technical aspects of RDBMSs. If you’re interested, consult any one of the many available textbooks that discuss RDBMS principles and technology in detail.

An RDBMS is a software system that manages relational databases. So, what’s a relational database?

In a typical spreadsheet program, columns and rows form a series of cells. If each column is headed by the name of a data attribute (CUSTOMER_NUMBER, PRODUCT, and QUANTITY_PURCHASED, for example) and each row has a single value for each attribute, you have the basics of a relational database table, as shown in this example:

12345             Vegetable soup    5
45678             Cooking oil       3
42973             Lawn fertilizer   2
81115             Blankets          88
81115             Vegetable soup    33

A relational database typically has many different tables — a CUSTOMER_MASTER table and a PRODUCT_MASTER table, for example, in addition to the table shown here, which you could call ORDER_DETAIL or WHO_ORDERED_WHAT.

You can combine information from across the various tables by joining those tables (making a match between tables, usually by looking for columns in two or more tables that are the same). For example, CUSTOMER_MASTER might contain the following rows of data:

12345             Mark Jones
45678             Daniel Michaels
42973             Karen Warner
81115             Susan Robinson

Joining these two tables, using CUSTOMER_NUMBER as the common attribute, might tell you that customer number 81115, named Susan Robinson, ordered 88 blankets and 33 cans of vegetable soup. (She must be going on a camp-out!)

From either of the individual tables, you have only bits and pieces of this information: You don’t know the customer’s name from looking only at the first table, for example, and you don’t know what products Susan Robinson ordered from looking only at the second table. The power of the relational database becomes evident when you join tables together.