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Introduction to SQL

The term SQL stands for structure query language. It consists of a set of command that humans use to instruct a computer to store, modify and retrieve information.

This posting and a few more to come will discuss the commands used to create/allocate storage space, how data is stored, modified, retrieved, erased and secured. The next few postings on the subject will also talk about why it is important for every business to acquire, store, retrieve, accurately interpret and secure such information.

Creating a Database:

Before any user data can be stored in what is called a "Relational database system, you need to create a space to store related information. This space is what is called a database, which is basically a collection of entities called tables and relationships among those entities.

To create a database, we use the following SQL command/statement:

 create database db_name;

where db_name is the name of your database.

Once you have a database, you can now start creating your data entities. I am assuming that you have already identified these data entities and attributes or features that are important to your business.

For example: You own a business that rent out books. Your business process requires you to keep track of which book is in stock, which is borrowed and by who, when you expect your books to be returned and so forth.

To be able to keep this information without having to duplicate the information every time a book is borrowed or returned, you will need to identify entities for which you want to keep information. The information in these entities will then only be recorded once.

In this case we know that we want to keep information about borrowers and information on every book that belong to your business. In addition to this, you want to keep information on the borrowing process itself; such as transaction date, when you expect the books to be returned, the condition of the book when borrowed and its return condition and so forth; which we consider to be attributes of the borrowing entity.

In SQL terms we would define the borrower entity as follows:

Create table entity_name(

            borrower data type,

            booknumber data type,

            borroweddate data type,

            returndate data type,

            borrowcondition data type,

            returncondition data type);

where data type is one of the following:
1) integer,
2) varchar
3) text and so fort...

Now that we have our first entity is created, we have to start adding data to it, this happens when for example somebody comes in to borrow a book. To add data to an entity, we use the insert command. This is expressed as follows:

 insert into borrowing(borrower,booknumber,borrowdate,returndate,borrowcondition,returncondition) 

values(value1,value2,value3,value4,value5,value6);

Note: the number and type of values have to match the specified fields or attributes, otherwise it won't work. In some cases you may skip specifying the attributes you want to add; if you do then you have to make sure the values match the order and type of attributes in that entity or table.

Now that you have information stored, how do you retrieve it? In other words say, you want to know who borrowed what book.

To do this you would use the select command or statement to request the information. Bellow is how you do it:

Select * from borrowing;

The statement above tells the database system to return everything that is stored in the table named borrower .

You can also choose just to get part of the information stored in the table. For example, if you only want to see the borrower and the date on which they borrowed the book, you would do the following:

Select borrower, borroweddate from borrowing

in which case, we are telling the system to give us data in the two fields/attributes (borrower and borroweddate).

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Introduction to SQL

Creating a Database:

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