Database Essentials - Relationships
(Page 3 of 9 )
Many of today's databases are "relational database management systems", referred to by the acronym RDBMS. A relational database is typically composed of multiple tables, which contain inter-related pieces of information. SQL, or Structured Query Language, allows you to combine the data from these tables in different ways, thereby allowing you to create and analyze new relationships.
The example you just saw consists of a single table. While this is fine by itself, it's when you add more tables, and relate the information between them, that you truly start to see the power inherent in this system. Here's an example:
# "names" table
+------------+-----------+----------+
| CustomerID | FirstName | LastName |
+------------+-----------+----------+
| 234673 | John | Doe |
| 734736 | Julius | Caesar |
| 1243 | Daffy | Duck |
+------------+-----------+----------+
# "addresses" table
+------------+------------------------------+---------------+-------+--------+---------+
| CustomerID | StreetAddress | City | State | Zip | Country |
+------------+------------------------------+---------------+-------+--------+---------+
| 234673 | 12, Some Street | Nowheresville | ZZ | 748202 | 56 |
| 734736 | The Palace Behind The Lake | Ancient Rome | NA | 0 | 0 |
| 1243 | Next Door To The Rabbit Hole | Toontown | TT | 98765 | 0 |
+------------+------------------------------+---------------+-------+--------+---------+
# "balance" table
+------------+----------------+
| CustomerID | AccountBalance |
+------------+----------------+
| 734736 | 6546867.67 |
| 234673 | 3275.99 |
| 1243 | 3.00 |
+------------+----------------+
Each of these three tables is an independent entity; however, they can be linked to each other by means of the CustomerID field common to all of them. For example, you could easily use the CustomerID 1243 to find out that user "Daffy Duck" lives in Toontown, and has a balance of exactly $3.00 in his account. Or you could go about it the other way: among all the customers with a balance less than $10.00 in their accounts, CustomerID 1243, aka "Daffy Duck", is the only one living in Toontown, TT.
These relationships are the fundamental basis of a relational database system. By making it possible to easily relate records in different tables to one another, an RDBMS makes it possible to analyze date in different ways, while simultaneously keeping it organized in a systematic fashion, with minimal redundancy.
Next: Invasion Of The Foreign Keys >>
More Administration Articles
More By icarus, (c) Melonfire
/ 7 
