Learning Outcomes
On completion of this chapter you will know
·
The concept of a database
·
The concepts and uses of terms
like table, record, field, record size, indexes and key fields
Introduction
Although we met a form of database already
when we were learning spreadsheets, it was not a proper database in the
strictest terms as defined in the IT field.
A spreadsheet is designed for the
mathematical manipulation of relatively small amount of mainly numeric data,
whereas a database is for the storage of large volumes of mainly non numeric
data and has very limited capacity for calculations. On the other hand a database has excellent
means of validating data and equally excellent reporting facilities, while the
spreadsheet has very limited capacities in those two areas.
Tables
The most basic unit of a database is a table.
A database can have as many tables as are required but in this introductory
course we shall look at a database containing only one table. Later on we shall look at multi table databases
but to start with we shall be looking at a database containing only one table.
Listing 1‑1
A very simple table is shown in Listing
1‑1
above. As you can see it’s a 5X7
grid. At a quick glance we can see that
it is a list of people’s names, addresses and ages. The very top row contains the names of the Fields in the table. Those are named Title, Surname, Name, Address and Age. The first row above
does not actually form part of the table and is not part of the data stored in
the table. It is in fact there as information to explain what is in the columns
below it. The actual data starts below
it in the row that contains the details about Harry Potter. This entire row is called a Record. A Table can have as many rows
as is required. The only limit is the
size of the disk that contains the original database. Each Record
is made up of Fields. A field is where a row and a column
meet. In our case the record is made up
of five fields. The names of the fields
indicate what type of data is to be stored there. Thus the Title field contains Mr,
the Surname field contains Potter, the Name field
contains Harry etc.
Although a table can contain as many
records as there is space for in the disk, each record must have exactly the
same number of fields. We cannot add an
extra field to one record and leave the rest unchanged. Of course if we realize that we need extra
information about a person such as his date of birth we can add that field to
the table – but it will be added to every record in the table, not just to the
one record we want.
Summary
A table is a logical structure for storing
data. The various data items we wish
to store are stored in Fields. A number
of fields make up a record. The structure
of each record must be exactly the same, in other words it must have the same
number of fields and in the same order. A
table can have as many records as are required and is only limited by the
size of the disk on which it is stored.
Exercise 0
1)
Table, field, database, record - put
the four words in logical order
2)
A database can contain only one
table. True or false?
3)
In a table different records
can contain different number of fields. True or false?
4)
The maximum number of records
in a table is 1,000. True or false?
5)
In Listing
1‑2 below
list all of the features that are wrong for a table. When finished design a correct version that
could become a table.
Listing 1‑2
|
Name |
Age |
Profession |
Marital Status |
|
|
|
John Smith |
34 |
Mechanic |
Teacher |
Married |
|
|
Michael |
James |
46 |
Driver |
Single |
|
|
Frank Alois |
27 |
Policeman |
Separated |
|
|