Easy SQL Queries Are in Reach

Structured Query Language or SQL is an easy to learn computer language for accessing and manipulating database systems. This is a language that works with programs such as Microsoft Access, DB2, Informix, and Oracle SQL Plus.

There are different versions of SQL; for example, a query written for Microsoft Access is different then a query written for Oracle, even though they produce identical results. To meet American National Standards Institute guidelines, all SQL queries include the SELECT, FROM and WHERE clauses.

There are several SQL editing and reporting packages available including Microsoft Access and Crystal Reports; however, for this tutorial, the query will be written for Oracle SQL Plus. SQL Plus is a command line interface that is bundled with the Oracle Database Client and Server. It is interactive or script driven and is used by database administrators and developers to access Oracle databases.

A table, the basic unit for a relational database, is a set of related information. Tables are then further separated into columns that hold a specific data type within the table. For example a data table of employee information would have data in columns of employee number, first name, last name and telephone number.

The first step to understanding how to use SQL to get information is to know what is available in a database. A schema is a listing of all of the data that is in a database and includes the table layout and a diagram that shows the relationship between the data.

In Oracle SQL plus, a user can see what columns are available by typing the command 'desc' and the name of a specific table. In this tutorial, one of the tables used is the 'employee_information table which has five columns. In the example below, entering DESC employee_information at the command line produces the following result.

SQL> DESC employee_information

EMP_NB

FIRST_NAME

LAST_NAME

TELEPHONE_NB

HOME_TOWN

There are three clauses that make up a SQL query: the 'SELECT'clause; the 'FROM' clause; and the 'WHERE' clause. Each clause provides the query with specific information on the data that is to be pulled.