Download Fields and Properties

A DATABASE AND ITS COMPONENTS
A database is an organized collection of data. It is kept to enable the user to
access information quickly and efficiently in the future. In order for this to be
possible the database needs to be carefully designed and data input must be
careful and accurate.
The smaller we make the items of data that we store the more powerful the
database can be.
A Database is composed of files - or in a relational database these are called
'tables'. A table represents an object or 'entity' about which we want to store
data. Step 1 is to decide on the entities in our database.
Each table contains whatever 'fields' we put into it. A field holds a single item of
information about an object or entity. For example for the entity STUDENT we
would want to record the STUDENT_ID so we would create a field with this name
and into it would put the Student_ID for each student. These pieces of data are
called 'attributes' and they are stored in the fields. We name the fields so that it
is easy to understand what they contain!
A record is the complete set of data on a particular entity - for example all the
items of data we store on one student such as ID, Name, Address, Tel No, Age,
Height etc.
A Relational Database (Access) is made up of a number of Tables which represent
the objects or entities.
Relationships are defined by the user between entity tables in the database
enabling many useful searches to be made for data in the database.
SETTING UP DATABASE TABLES
1. Identify the 'entities' that we need in the database
2. We set up a table for each entity
3. We decide on the 'fields' that need to go in each entity table
4. For each field we must decide what type of data it will hold and any special
properties that the data must have
DATA TYPES
The types of data that can be stored in a field must be defined and can belong to
one of the following categories
Data Type
Description
Text
A character string with a maximum length of 255 characters
Memo
A character string with a maximum length of 64,000
characters. Usually used for descriptions of items. Cannot
be used as a Primary Key
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Database Field Properties
Data Type
Description
Number
Various types of numbers. Double (15 places, 8 bytes) Single
(7 places, 4 bytes) Long Integer (-2.1 billion to +2.1 billion,
4 bytes, no decimals) Integer I-32 to +32 thousand, 2 bytes,
no decimals) and Byte (values 0 to 255, 1 byte, no decimals)
AutoNumber
The database will automatically generate numbers for this
field. Usually unique identifier for primary key purposes
Yes/No
Used to store logical results from True/False results. Be
careful as –1 stands for Yes and 0 stands for No
Date/Time
Used to store dates and times. Format can be changed
Currency
Used for storing numbers (not necessarily in money format).
Can hold 15 places to the left of the decimal point and 4
places to the right. Takes up to 4 bytes
OLE Object
Used for linking other files to the database such as
spreadsheet
Hyperlink
Used for storing web addresses
FIELD PROPERTIES
Field properties are set to control how the data in a field can be entered and
displayed and in some cases to limit the type of data that can be entered. When
setting up a table you need to go through the list below for all fields
Field Name
Each field name must be unique
A field name can have a maximum of 64 characters
A field name can be any combination of letters or characters or spaces or special
characters EXCEPT
Exclamation mark
!
Accent grave
`
Full stop
.
Brackets
() or {} or []
Do NOT begin a field name with a space
Although it is allowed to include spaces in field names it is much BETTER not to.
Use CustomerName of for clarity use the underscore symbol is often used to
separate words ie Customer_Name
Data Type
Select the data type from the list given previously
Field Size
Allows you to limit the number of characters that can be entered
Primary Key
Uniquely identifies a record in a table. A table must have a Primary key defined
before it is saved. Ensures that no two records in the same table can contain the
same data in the key field. (Atomic data value)
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Database Field Properties
Input Masks
Input masks are used to format data and provide some control over what values can
be entered in a field. Input masks can be used in tables, and in combo boxes and
text boxes in forms. Mostly they are used with Text and Date/Time field types
Examples are if we want all the names in our database to have initial capitals but
the rest of the names in lowercase.
Use of Input Masks – To ensure the lettering in the database is consistent. It does
not matter how we type in the data it will be displayed the same in all records
>?<???????????
Result
>?????????????
Result
To display an initial capital letter and the rest small letters
Dublin
Aberdeen
To ensure that all the letters appear in capitals
DUBLIN
ABERDEEN
The ? represents any letter from A-Z (entry optional)
The L represents any letter from A-Z (entry required)
With ? if a name is shorter than the number of characters specified for the field the
data is accepted. If the L is used then for each L in the input mask there must be a
letter in the field entry (Not a great idea for people’s names since they are all
different lengths
A couple of other useful characters to know are:
0
9
#
+
[a digit 0-9, entry required, + and – signs not allowed]
[a digit or space, entry not required, + and – signs not allowed]
[a digit or space, entry not required, blanks positions converted to spaces
and – allowed]
Input masks can be used for formatting but also to ensure that the data entered
follows a particular pattern so can help with Data Integrity. Input masks are used
for telephone numbers, Postcodes, Social security numbers etc. Input masks do not
work with all data types. An input mask for a phone number can limit the number
of digits that are entered and ensure that only numbers and not letters are
entered. A full list of input mask characters is given in Access/Help
Validation Rules
Validation rules are to help to prevent input errors by limiting the data that fields
accept. For example if a company had commenced business in 1999 then a
validation rule could prevent them having hired any employees before 1999. This
would force the entries to be after the date specified
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Database Field Properties
The mathematical operators are:
<
>
<=
>=
<>
less than
greater than
less than or equal to
greater than or equal to
not equal to
<#1/1/99#
>#1/1/99#
Examples of what is typed in the Field Properties
Rule
>=#1/1/99# and <= #14/2/04#
=”UK” Or=”USA” Or=”IRL”
<=Date()
<>#25/12/04#
>=100
=”Male” or “Female”
What it means
date must be between 1/1/99 and 14/2/04
only the data shown will be accepted
Today at the latest
Not Christmas Day
Must be greater than or equal to 100
Must enter the words Male or Female
Validation Text
This is a message typed by the Database designer in the appropriate place in the
field formatting section, to tell the person entering data that they have made an
error and the type of entry that is allowed
Required Property (Yes/No)
A field can be set that it cannot contain a Null value (cannot be empty). The key
field or primary key needs to be set for required - Yes
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Database Field Properties
Default Value
You can set the default value or text for a field to save time inputting data later.
Obviously you can overwrite the default value if it is not what is required in a
particular record
OTHER DATABASE DEFINITIONS
Referential Integrity
Prevents users from entering inconsistent data in the database. These rules are
applied when you are creating relationships between two tables. It ensures that
the relationships in the database are enforced. The term Data Integrity means
that the data in the database is correct, consistent and up-to-date at all times.
For example if we have a Properties table and an Owners table, we cannot list an
Owner in the Properties table if he/she does not exist in the Owners table.
Normalisation
This makes the tables as efficient and compact as possible to eliminate error and
confusion. There are a number of normalization rules eg
(i)
Avoid repeated information – for example if you are recording customer
orders it would be very wasteful of time and space to record their name and
address etc each time they placed an order. It is better to assign each
customer an ID and put that information into a separate table and then just
refer to their ID in the Orders table
Foreign Key
A non-primary key field which is linked to the primary key of another table
OTHER DATABASE OBJECTS
Tables are only one of the objects we find in an Access database. Some of the
others are briefly described here.
Forms
A Form is a data entry tool. Example might be an Order form. When entering data
using a form you see only one record at a time. When you are entering data in a
table you can see a number of records at once. Tables can be easily converted into
Forms using Autoform or by designing your own. A database can be updated
through a form or a table.
Queries
Are a way of sifting or filtering out information you do not want to see and also
selecting information/data that you do want to see ie ‘When did we employ Rose
Townsend?”. The results of a query can be saved or discarded. Queries can be
used as a basis for a report.
Reports
Used to present data and information. There are a number of layout options. They
can use tables or queries as their source. Data can be organized so that it is easier
to understand.
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Database Field Properties