Complex Multi Field Key Question

[Tim Whitley]

I have a key that needs to be based upon 3 fields: "Service", "Cycle" and
"Rate Code". I have a query that contains each of these items along with
relevant financial data driving the rates for clients. I am pulling this
data via ODBC (not sure if that matters).

I need to tie this "multi field Key" to 2 tables "Account" and
"Subscription". These two tables have a one to one relationship between them
(Account ID). The "Account" table has the "Rate Code" and "Cycle" fields,
while the "Subscription" table has the "Service" field. In essence, the
"Service", "Cycle" and "Rate" fields are needed to determine what the rate in
dollars each customer is currently on by a relationship to the "Current Rate"
field in the "Subscription" table.

For example, the customer may be on a SUNDAY subscription, MONTHLY billing
cycle and a 80 rate code which would indicate they are charged 1.00 per
issue.

I have tried linking the Rate Query fields mentioned above to the two tables
and Access gives me an outer join error. I am not that experienced and was
hoping for some help from the experts.

Thanks in advance.

[Tom Wickerath]

Hi Tim,

I have a general dislike for combined field primary keys. You should be able
to accomplish the same result by using a multifield unique index. This would
allow you to add a meaningless autonumber primary key, which you can then use
for joining to other tables. (In your 1:1 relationship, one table would need
a number / long integer data type to join to the autonumber; you cannot join
two autonumber fields). Here is an article that you might be interested in
reading:

The case for the surrogate key (see article # 4)
http://www.access.qbuilt.com/html/articles.html

Here is a posting from Utter Access that I bookmarked a few years back. I
think the author makes some reasonable arguments on this topic:

http://www.utteraccess.com/forums/showflat.php?Cat=&Board=accessfaq&Number=247916&page=0&view=collapsed&sb=5&o=&fpart=1

Good Luck,

Tom Wickerath, Microsoft Access MVP

http://www.access.qbuilt.com/html/expert_contributors.html
http://www.access.qbuilt.com/html/search.html
__________________________________________

[Tim Whitley]

Thanks for the insight, Tom.

[RoyVidar]

Tom Wickerath wrote in message
<8E52A6F4-3705-4C2C...@microsoft.com> :

> Here is a posting from Utter Access that I bookmarked a few years
> back. I think the author makes some reasonable arguments on this
> topic:
>
> http://www.utteraccess.com/forums/showflat.php?Cat=&Board=accessfaq&Number=247916&page=0&view=collapsed&sb=5&o=&fpart=1

From that article:

"A very tempting, but incorrect, Primary Key for tblJunction is a
composite one consisting of the two foreign key fields ID1 and ID2.
However, the Primary Key, as always, should be a single-field,
preferably AutoNumber."

Composite primary key incorrect???
A primary key should be single field???

I haven't been able to find anything supporting that in my database
literature. Could anyone point out any references to serious database
literature supporting this?

--
Roy-Vidar

[Tom Wickerath]

Hi Roy,

> Composite primary key incorrect???
> A primary key should be single field???

This is the opinion of the person who wrote the article, although it is not
shared by all developers.

> I haven't been able to find anything supporting that in my database
> literature. Could anyone point out any references to serious database
> literature supporting this?

I did some research tonight, looking at LOTS of books in my library and a
few web-based resources. Here is what I found, which may or may not fit your
definition of "serious database literature":

From Access 2002 Desktop Developer's Handbook
by Paul Litwin, Ken Getz and Mike Gunderloy (page 88)

"The decision as to which candidate key is the primary one rests in your
hands; no absolute rule dictates which candidate key is best. Fabian Pascal,
in his book SQL and Relational Basics, notes that the decision should be
based on the principals of minimality (choose the fewest columns necessary),
stability (choose a key that seldom changes), and simplicity/familiarity
(choose a key that is both simple and familiar to users).

From SQL Queries for Mere Mortals
by Michael Hernandez and John Viescas (page 38)

"Define a Simple Primary Key when you can because it's more efficient and is
much easier to use when establishing a table relationship. Use a Composite
Primary Key only when it's appropriate (for example, to define and create a
linking table)."

Designing Effective Database Systems
by Rebecca M. Riordan (page 33)

"Some people are under the impression that composite keys are somehow
incorrect, and that they must add an artificial identifier--either an
identity or autonumber field--to their tables to avoid them. Nothing could be
further from the truth. Artificial keys are often more convenient, as we'll
see, but composite keys are perfectly acceptable."

Database Normalization Tips
by Luke Chung
President of FMS
December 2001, last updated March 2005

http://www.fmsinc.com/tpapers/genaccess/databasenorm.html

In general, a key field should have these characteristics:

Should be One Field
It is possible to define multiple fields as the key fields of a table, but a
single field is preferable. First, if multiple fields are necessary to define
uniqueness, it takes up more space to store the key than a single numeric
field. Second, additional indexes on the table also have to use the
combination of the key fields which takes up more space than if it were a
single field. Finally, identifying records in the table requires grabbing a
combination of fields. Far better to have a CustomerID number than a
combination of other fields to define a customer.

So, perhaps the author of the Utter Access article was a bit strong in his
language, but his recommendation is supported by others. The point that he
was trying to emphasize was that it is easier to work with a simple key
versus working with a composite key. As always, YMMV (Your Mileage May Vary).

Hope this helps.

Tom Wickerath, Microsoft Access MVP

http://www.access.qbuilt.com/html/expert_contributors.html
http://www.access.qbuilt.com/html/search.html
__________________________________________

[Roger Carlson]

The assertion in the article (that it is *always* incorrect to have a
multiple field primary key for a junction table) is rubbish. If it is a
*pure* junction table (that is, the table's ONLY use is to implement a
Many-to-Many relationship), then the *preferred* way is to make the primary
key a combination of the two foreign keys (especially if these foreign keys
relate to autonumber primary keys in the base tables). By adding a
surrogate key (autonumber primary key), you are simply maintaining an
additional index for no reason.

That said, if the junction table participates in any *other* relationships,
it is useful to create a surrogate key and create a unique index made up of
the two foreign keys. Even so, it's not mandatory (see Problem 3 below).

Let me discuss the author's three problems:

Problem 1a:
If you want to delete a record from your junction table, you'd have to
supply values for both fields, rather than a single value. So you would
have to do this:
Currentdb.Execute "DELETE FROM tblSudentClasses WHERE StudentID = " &
me.cboStudent & " AND ClassID = " & me.lstClasses.

instead of this:
Currentdb.Execute "DELETE FROM tblStudentClasses WHERE StudentClassID = " &
me.lstClasses

Perfectly true. But his assertion that it is faster and more effiecient is
just not so. It is, of course, a tad more difficult to code, but in terms
of execution, the the difference would be negligible. In fact, since Access
stores primary keys as a clustered index, it may even be faster! You'd have
to do a benchmark to test it, but to a human difference will be
indistinguishable.

Problem 1b:
"if you ever decide to allow a student to take a class multiple times you
have to change this code to include your new field of the PK (like class
date)"
This is true, but if it is ever possible that a student can take a class
multiple times, then you should have taken that into account in your initial
database design. And even if you didn't, and you had to make this change,
you'd still have to modify your table structure because you'd want to store
this date and the junction table would be the logical place. And you would
*still* want to maintain real-world uniqueness in your junction table, which
would require a unique index on the three fields anyway.

Problem2:
"What if you wanted to select a record with a multi-field PK from a combo
box?
John Calculus
John Chemistry
Jane French
Jane Biology"

This case is very true, but does not apply to the Junction table. Remember,
we are talking about foreign keys that are related to surrogate keys in the
main tables. You would never have values as above. This argument is
pertinent to the notion of using multiple field primary keys in general, but
not the specific case of a junction table.

Problem3:
"Now you have tblTests with TestID and want to create tblStudentClassTests
to show what each student got on each test for each class."

This is the case above where the junction table is not a pure junction
table, ie one that participates in another relationship. In this case, it
IS preferred to create a surrogate key for the primary key, but it is not
MANDATORY. His contention is that you'd HAVE to store values from both
fields in the composite key in the related table is just plain wrong.

Let's take the case above. You've created your database with
tblStudentClass as a junction table. Later, you are asked to modify the
database to store test scores. You create tblTest and realize this is also
a M:M relationship that requires its own junction table
(tblStudentClassTests). Rather than relating the table on the primary key
of tblStudentClass, you'd create an autonumber field and add a Unique Index
on it (call it StudentClassID). The you could relate your table on this new
field, rather than the primary key. While we usually create relationships
on primary key/foreign key, it doesn't have to be so. Any field with a
unique index will work. (Note: an autonumber field with a unique index is
also guaranteed to be non-null).

Now I'm not saying I'd design it like that from scratch. If I *knew* a
junction table would participate in other relationships, I *would* create a
surrogate key. However, if I found myself in the situation where I had to
add a new relationship after the fact, I would NOT have to resort to
relating on multiple fields.

All of this is not to say I'm a fan of Natural Keys. I'm not. I use
Surrogate Keys in almost every table EXCEPT in pure junction tables.

--
--Roger Carlson
MS Access MVP
Access Database Samples: www.rogersaccesslibrary.com
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"RoyVidar" <roy_vid...@yahoo.no> wrote in message
news:mn.951a7d649...@yahoo.no...

[RoyVidar]

Tom Wickerath wrote in message

<9847FA46-F9B6-474C...@microsoft.com> :

Thank you Mr. Wickerath,

Your description "perhaps the author of the Utter Access article was a
bit strong in his language" is something I can agree with ;-)

The point he was making, was that composite primary keys are incorrect
for junction tables. I'm afraid I'm still convinced there's little or
no
support in database literature for that.

Here's a quote from one of my books on primary keys in general
(Database
Systems, C.J.Date, sixt edition, 1995, page 79)
"The primary key is a unique identifier for the table - that is, a
column or column combination with the property that, at any given time,
no two rows of the table contain the same value in that column or
column
combination".

--
Roy-Vidar

[RoyVidar]

Roger Carlson wrote in message <et4BK48Y...@TK2MSFTNGP04.phx.gbl>
:

Mr. Carlson,

thank you very much for your clear and detailed post, to which I agree
completely!

One additional information, re problem 2. When working with a combo
based on a table with composite primary key, set the BoundColumn
property of the combo to 0.

The first thing to notice, is that when one select John Chemistry in
the
mentioned sample - then John Chemistry will remain selected. Why?

When the bound column property is set to 0, the listindex is what one
address when assigning or retrieving the combos value property, which
is
unique. So, the article is true that the combo "just wants to know what
record number you're talking about" ;-)

Then it should be just some smallish programming to assign and maintain
the correct combo selection.

--
Roy-Vidar

[Tony Toews]

RoyVidar <roy_vid...@yahoo.no> wrote:

>"A very tempting, but incorrect, Primary Key for tblJunction is a
>composite one consisting of the two foreign key fields ID1 and ID2.
>However, the Primary Key, as always, should be a single-field,
>preferably AutoNumber."
>
>Composite primary key incorrect???
>A primary key should be single field???

This is a religious argument. <smile>

Tom Ellison argues quite effectively for composite primary keys.
However the Access UI needs to be tweaked to better support composite
primary keys.

Using the term incorrect would be a bit strong IMO. It's more a
matter of taste.

That said I only every use autonumber primary keys. I will frequently
use non duplicate multi-field indexes to restrict what is entered. Ie
only one person can register for a given course.

Tony
--
Tony Toews, Microsoft Access MVP
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