Wednesday, January 12, 2011

Databases - Introduction

In the information age (which started in 1940 or so when the US census used the first computer), a database is referred to as a silo of information which has data stored in a particular way. This makes stored data relevant to employees in order to make business decisions. Databases are a specific type of model. Models are any storage repository (server) which stores data whether it is a relational database, object-oriented database, flat or just files.

In this article, we confine ourselves to relational databases like MySQL, MS SQLServer20xx, Oracle, Sybase, MS Access and HSQLDB. These are the most common relational databases currently in use.

Non-Relational Databases
Before we move onto relational databases we will first discuss non-relational ones. These demanded a lot of sort to find code by programmers in order to retrieve the data to the users. Consider the US phone book and the number 307-456-7892. In order to search the phone book to retrieve who is associated to this phone number, the code (pseudo-code) would look something like:

.... set up code
do while (!eof(PhoneBook)) {
if (recordPhoneNumber EQUALS '3074567892') then
get out of the DO WHILE;
... grab the record's last name and first name and send it back.

This means that for every phone number, we will go through reading the file. Of course, if we had the file sorted by phone numbers a priori then it will be very easy to retrieve the name. Now, if we were given the last name of the individual, then retrieving the phone number would hold the same logic. As you can already see this is CPU (Central Processing Unit) intensive and there are better ways of storing data and retrieving them in less time.

Object Oriented Databases
Object Oriented(OO) Databases store objects. Objects have attributes and methods. These can be retrieved with OO languages like SmallTalk and Java, C++. An article [1] in the reference section gives a short description of OO databases including their advantages and disadvantages. I urge the reader to skim through said article to get familiar with OO databases.

Relational Databases
A relational database is two-dimensional and stores data in rows with the columns being attributes. It is akin to a spreadsheet. A spreadsheet (or a file) which stores particular data by the last name, first name, address and the telephone number of individuals is referred to as a table and each row in the spreadsheet is generally called a record.

Consider our example in the flat file section where we need to retrieve the person associated with a telephone number. Now the pseudo code is reduced to a query in Structured Query Language (SQL) as:

SELECT * FROM person WHERE telephoneNumber = 3074567892;

The database engine then searches the table to find the person for the record with the telephone number 3074567892 and returns the record. Now if there is no record matching the number then it will return zero records.

In a database, a table can have index files which are files that sort the table on a specified column or concatenation of columns. When the query is sent to the table then the index file (in our example, the telephoneNumber Index file) will be used if it exists. If there is no index file then we are back to a full scan. Now, if we were given the last name and were asked to retrieve the phone number, then the query is basically the same except for the where clause i.e.

SELECT * FROM person WHERE lastName ='COOK';

and the database engine will utilize the lastName index file for the above query.

Primary Id
Each table stores information regarding a primary unit in a structured way. For instance, a person can have many types of email addresses, business addresses, physical addresses, mail box addresses and sometimes a permanent address. This automatically lets a database designer think in terms of having a table which stores addresses and a table called "say person" which stores the last name, first name etc of the individual.

To join the person table and the address table we define a unique identification for each person such as SSN, DL # or just a sequence. This is then stored in both tables and we are then able to join the two tables to retrieve information of the person and their addresses. In other words, we relate the person table to the address table on the key sequenceNumber. Now the query will look like this:

SELECT * FROM person, address WHERE person.sequenceNo = address.sequenceNo AND person.telephoneNumber = 3074567892

This code will return all the records from both tables; that is if they match in both tables for the specific telephone number.

Index files are created on primary keys generally to speed the process of querying (searching).

Now that the storage of data (models) has been introduced, we move onto other topics in programming and analysis. We now move into some SQL for creating, inserting data and also joins of tables in Databases Basics.


[1]An Introduction to the Object Oriented Databases article.