Jeff has given you a good solution for 'normalizing' your database. As
background you might find the following brief summary of what normalization
means helpful. Its not meant to be an exhaustive description of the subject,
and doesn't deal with some of the more advanced aspects, but I hope it helps
you gain an understanding of the main principles:
"Normalization is the process of eliminating redundancy from a database, and
involves decomposing a table into several related tables. In a relational
database each table represents an entity type, e.g. Contacts, Companies,
Cities, States etc. and each column in a table represents an attribute type
of the entity type, e.g. ContactID, FirstName and LastName might be attribute
types of Contacts and hence columns of a Contacts table. Its important that
each attribute type must be specific to the entity type, so that each 'fact'
is stored once only. In the jargon its said that the attribute type is
'functionally dependent' solely on the whole of the primary key of a table.
To relate tables a 'referencing' table will have a foreign key column which
makes the link to the 'referenced' table, e.g. a Contacts table might have a
CompanyID column as a foreign key, while a Companies table has a CompanyID
column as its primary key. Consequently no data other than the CompanyID
needs to be stored in a row in the Contacts table for all the company
information for that contact to be known; its available via the relationship
and can be returned in a query joining the two tables on the CompanyID
columns.
Similarly the Companies table might have a CityID column and the Cities
table a StateID column. If its an international database the States (or more
generically Regions) table would have a CountryID referencing the primary key
of a Countries table. So via the relationships, simply by entering (in
reality this would be selected from a list of Companies in a combo box, not
typed in)a CompanyID in the Contacts table the location of the contact's
company is also known. Redundancy, and therefore repetitive data entry is
avoided.
To see how a database is made up of related tables take a look at the sample
Northwind database. Its not completely normalized in fact (deliberately so
for the sake of simplicity) but illustrates the main principles of how tables
representing entity types relate to each other. An example of its lack of
proper normalization can be found in its Customers table. You'll see that
this has City, Region and Country columns so we are told numerous times that
São Paulo is in SP region (as is Resende) and that SP region is in Brazil.
Not only does this require repetitive data entry, but more importantly it
opens up the risk of 'update anomalies', e.g. it would be perfectly possible
to put São Paulo in California in one row and California in Ireland! Proper
normalization as I described above would prevent this as the fact that São
Paulo is in SP region would be stored only once in the database as would the
fact that SP region is in Brazil and California is in the USA.
An example of what at first sight might seem to be redundancy, but in fact
is not, can also be found in Northwind. The Products table and the
OrderDetails table both have UnitPrice columns. It might be thought that the
unit price of a product could always be looked up from the Products table, so
its unnecessary in Order Details. However, the unit price of a product will
change over time, but each order needs to retain the price in force at the
time the order was created. Consequently a UnitPrice column is needed in
both tables; that in products holds the current price and is used to get the
value for that in Order Details (code in the ProductID control's AfterUpdate
event procedure in the Order Details Subform does this), which then remains
static when the current price (in products) changes. In each case UnitPrice
is functionally dependent on the key of the table, so there is no redundancy"
Ken Sheridan
Stafford, England
