 |
 What
does the term "relational" mean?
A
relational database, as opposed to a flat file
database, can link two or more tables together.
Fields from these tables can be displayed together
on the same form or report, almost as if they
belonged to the same table. When you relate tables
in Alpha Five, the file that defines that relationship
is called a set. Remember that sets do not themselves
contain data, their tables do.
I'm
not sure whether to use a Set or a Table Lookup.
What are the advantages and disadvantages of
each?
A
relational set is efficient because it eliminates
the need to duplicate information in multiple
records or in more than one table. It also allows
you to put fields which are only necessary for
a small percentage of records into a small, secondary
table. Sets save disk space. When you need to
update information, sets save you from having
to go to several different places to make changes.
Some
users, however, are more comfortable with a table
lookup, in which the values from the lookup table
are actually filled in and stored in fields in
the first table.
One
subtle but important difference between lookups
and sets is that lookup values will not change
once filled in on a particular record, while
sets always display current information from
child table fields. If you use a lookup to fill
in unit price on your invoice table and you change
your prices, you can go back to last month's
invoices and see exactly what you charged customers
at the time you sent them the invoices. On the
other hand, you may want to use a set to link
your price table to your product catalog table
so that you always have access to current prices.
How
do I decide which table should be the primary
table in my set?
In
designing your set, consider the following. In
a set you can access all of the records in the
primary table, but you only have access to those
child table records that match a parent record.
If you link a third table as a child of the second
table, you will only have access to a record
in the third table if a matching record exists
in both its parent (the second table) and the
primary table.
You
have the greatest degree of control over the
primary table of the set, especially with regard
to indexing. There is no limit to the number
of sets in which you can include a particular
table. To serve all your needs, you may wish
to flip-flop your set tree. Create one set with
table A as the primary and table B as the child.
Create another set using the same tables, with
B as the primary and A as the child.
I
made a set, but I can only see data from the
primary table. What happened to the child records?
One
possibility is an improperly defined link. If
the parent and child records aren't matching
up at all, this may indicate that and error in
selecting the linking fields or that common fields’ data
differs in some way.Another possibility is a
corrupted index. When you add a table to a set,
Alpha Five builds an index on the linking field
of the child table. Try rebuilding the child
table's indexes.
Is
it possible to link together in a set tables
whose common fields do not contain exactly the
same data?
In
the Set Editor, Alpha Five allows you to use
an expression to define what constitutes a match
between a parent record and a child record. A
good illustration of this is a set in which the
primary table contains a shipping address, including
a five digit zip code. The child table contains
UPS shipping zones which are based on the first
three digits of the zip code. The linking index
uses the child table's three-character zip field.
To match up the correct UPS shipping zone with
the first three characters of the zip code field
in the parent table, use the following common
field/expression: SUBSTR(ZIPCODE,1,3)
Proctor &
Peake, Inc.
9620 NE 2nd Avenue, Suite 209
Miami Shores, FL 33138
305-751-1181 / 1-800-943-6031
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us
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