Datasheet

Part I SQL Basic Concepts and Principles

them into tables (think of a ﬁle cabinet) alleviates the problem somewhat but does not remove

the major obstacles: data redundancy (the same information, or even worse, supposedly the same

information, might be stored more than once in different ﬁles), slow processing speed (‘‘I know

it was there somewhere ...’’), error-prone storage, and retrieval. Moreover, it required intimate

knowledge of the database structure to work at all — it would be utterly useless to search for,

say, orders information in the expenses ﬁle.

For example, you could design a ﬂat database system for an order entry system that gathers

information about customers, orders the customers have placed, and products the customers have

ordered. If data is accumulated sequentially, your ﬁle will contain information about customers,

then orders and products, then about some new customer, and so on — all in the order the data

is entered (see Table 1-2). Just imagine a task of extracting any meaningful information from this

mess, not to mention that a lot of the cells will remain empty. (For ‘‘Ace Hardware,’’ what would

you put in the Quantity column; or for ‘‘Nails,’’ what would you put in the Address column?)

Dissatisfaction with these shortcomings stimulated development in the area of data storage-and-

retrieval systems.

Excel is often used to create ﬂat ﬁle databases.

Hierarchical databases

The concept of a hierarchical database has been around since the 1960s and — believe it or

not — it is still in use. The hierarchical model is fairly intuitive. As the name implies, it stores

data in hierarchical structure, similar to that of a family tree, organization chart, or pyramid.

You could visualize a computer ﬁle system as it is presented through some graphical interface.

The most popular hierarchical database product is IBM’s Information Management System (IMS),

which runs on mainframe computers. First introduced in 1968, it is still around (after a number

of reincarnations), primarily because hierarchical databases provide impressive raw-speed perfor-

mance for certain types of queries.

TABLE 1-2

Sample Flat File Records

Name Type Address Price Quantity

Nails Product n/a 100 2000

Ace Hardware Customer 1234 Willow Ct Seattle, Washington n/a n/a

Cedar planks Product n/a 2000 5000