Once either the User or System data source has been configured we can begin creating programs utilizing SQL to access a databases. The reference to SQL can either be pronounced S Q L or Sequel as tribute to the first relational database language actually known as Sequel. SQL (Structured Query Language) is a language standard for querying and manipulating databases. It allows for programmers to easily switch between various databases by standardizing the commands needed to perform the necessary tasks. SQL is comprised of fairly basic, straightforward commands that can be used to accomplish complex events. The language is broken up into threeparts, DDL (Data Definition Language) which creates, modifies, and deletes tables, DML (Data Manipulation Language) which works with the actual data contained in the databases, and DCL (Data Control Language) which controls security on database objects. SQL is a case-insensitive language, but most of the time the SQL commands will be coded in uppercase letters to distinguish it from the data and surrounding code.

Here is a simple command that will pull only the first name and last name of the students in a particular database.

SELECT firstname, lastname FROM students;

If we wish to qualify the list so that only students from Omaha are returned we add the WHERE command. This selects all the students from the database WHERE the city field is Omaha. The ORDER BY command will place the selected entries in alphabetical order first by their last name and then by their first.

SELECT firstname, lastname FROM students
    WHERE city=Omaha
    ORDER BY lastname, firstname;

Before we can retrieve data from the database, we must have data populating the database. Here is a simple command for inserting data into the SQL database:

INSERT INTO Students (lastname, firstname)
    VALUES ('Smith', 'John');

The values of John and Smith are placed in the Students database in the lastname and firstname fields. If the values of "John" and "Smith" were reversed they would not be placed in the correct fields in the database. The field order must correspond to the value order.

Deleting data from the database is an easy task as well.

DELETE FROM Students WHERE GradDate=1999;

UPDATE Students
    SET firstname = 'Chuney'
    WHERE lastname = 'Johnson';

In this instance someone had mistakenly misspelled Chuney’s name as Shotney. To correct this error an update needed to occur as evidence by the UPDATE SQL command. This only works, however, if there is only one member in the database with the last name of Johnson. If there are multiple instances of "Johnson" in the database then it would update all the first name fields of all the Johnsons. To make sure that this type of error does not occur a primary key field must be identified. A primary key field is a field that can only correspond to one record. A good primary field key to set up in the case of a student database would be the Social Security field.

The commands that control creating databases, deleting databases, deleting all the information contained in a database are also very simple SQL commands.