Primary Keys

[LurfysMa]

Most of the reference books recommend autonum primary keys, but the
Access help says that any unique keys will work.

What are the tradeoffs?

I have several tables that have unique fields. Can I use them as
primary keys or should I define an autonum primary key?

One table has information about the 50 states in the US. The table
looks like this:

State Capitol Date Admitted
Alabama Montgomery December 14, 1819
Alaska Juneau January 3, 1959
Arizona Phoenix February 14, 1912

Since the state names are unique, is there any reason not to make that
field the primary key?

Another table has to do with grade school multiplication tables. This
is a simple table something like this:

Factors Answer
1x1 1
2x1 2
2x2 4
3x1 3
3x2 6
3x3 9
...
12x1 12
12x2 24
...
12x12 144

Here again, the Factors field (a text field) is unique. Is there any
reason not to use it as the primary key?

In both cases, the tables are small, so adding another 4-byte field is
no big deal, but I'd like to keep the tables as simple as possible
unless there is some downside.

Thanks

 

[Guest]

Using a column that is guaranteed to be unique, as in your examples, will
usually work. The one place you would consider using an autonumber is if you
expected to have to change the other key value. I don't expect any states to
change their names any time soon, so it's probably ok there. In your other
example, is there any risk that you might change the values in the factors
field? On the other hand, even if you did need to change something, a
cascading update relationship would still accomodate this.

Bottom line: if you have a candidate field that, by its nature, is
guaranteed to be unique, use it.

Barry

 

[Amy Blankenship]

In my opinion, you're asking for trouble if you ever show the user the
primary key or if you might ever want to edit that information. Since
primary keys are normally the way you establish relationships, you don't
want them to ever change once a record has been created. Even though you
may think the key value won't change, typos have been known to happen.
Also, number fields take up less space in the database and primary keys, as
the source of the relationship, are typically repeated over and over in many
tables.

Therefore, I always use autonumbers. Other opinions vary.

HTH;

Amy

 

[RoyVidar]

Most of the reference books recommend autonum primary keys, but the

Access help says that any unique keys will work.

What are the tradeoffs?

I have several tables that have unique fields. Can I use them as
primary keys or should I define an autonum primary key?

One table has information about the 50 states in the US. The table
looks like this:

State Capitol Date Admitted
Alabama Montgomery December 14, 1819
Alaska Juneau January 3, 1959
Arizona Phoenix February 14, 1912

Since the state names are unique, is there any reason not to make
that field the primary key?

Another table has to do with grade school multiplication tables. This
is a simple table something like this:

Factors Answer
1x1 1
2x1 2
2x2 4
3x1 3
3x2 6
3x3 9
...
12x1 12
12x2 24
...
12x12 144

Here again, the Factors field (a text field) is unique. Is there any
reason not to use it as the primary key?

In both cases, the tables are small, so adding another 4-byte field
is no big deal, but I'd like to keep the tables as simple as possible
unless there is some downside.

Thanks

I'd recommend you to take the time to use your favourite search engine
for the terms like "natural vs surrogate primary key". Such search
will
probably list some of the pros and cons, in addition to hours of fun
;-)

Basically, some favours usage of surrogate keys (Autonumber), others
favours natural keys, which represents "things" having a business
meaning, and which can also be a combination of fields. Some (including
me) will use both, based upon the requirements. For state, I'd probably
use the two letter code.

Just be aware - for some this isn't just a matter of preference, it's
religion to a degree thats close to fanatism.

Just be sure that if you decide upon surrogate key (Autonumber), then
remember that this will not ensure the integrity of your data! It will
only ensure that each record has a unique number. Say in a table where
you have a unique field, but you decide to add an Autonumber field for
primary key, you will need to also add a unique index on the "natural
key" field in addition to the primary key index on the Autonumber
field,
else you'll risk dupes.

 

[LurfysMa]

In my opinion, you're asking for trouble if you ever show the user the
primary key

Why is merely showing the user the primary key a problem?

 

[Guest]

Primary keys build an index and if you are concerned with database size then
two letter abbreviation for the state would be a smaller index.

I have another case for your in that I have to keep training certifications
and occupational examination records on personnel. But they keep changing
departments, names, employee code when migrating to different subcontractors,
etc. I set up an alias table that will have all changes and you can see that
the database records reflect that Jane Doe, now married to Bill Smith, had
hearing exam last year. The data matches paper records. There is a new
alias record for every change and the front/top displays the latest always
with a subform showing the current and all previous aliases.

 

[LurfysMa]

Primary keys build an index and if you are concerned with database size then
two letter abbreviation for the state would be a smaller index.

Since there are only 50 states, the savings would be negligible even
if we annex Canada one day! ;-)

I was more interested in usage and reliability tradeoffs.

I have another case for your in that I have to keep training certifications
and occupational examination records on personnel. But they keep changing
departments, names, employee code when migrating to different subcontractors,
etc. I set up an alias table that will have all changes and you can see that
the database records reflect that Jane Doe, now married to Bill Smith, had
hearing exam last year. The data matches paper records. There is a new
alias record for every change and the front/top displays the latest always
with a subform showing the current and all previous aliases.

Sounds messy...

 

[Amy Blankenship]

Because by user I mean people other than the developer who might be charged
with maintaining your data. When you show something to that type of user,
you lay it open to being changed. Here's a full discussion of the issue
http://www.dbpd.com/vault/9805xtra.htm

 

[Larry Linson]

Since there are only 50 states, the
savings would be negligible even
if we annex Canada one day! ;-)

Your mileage may vary, but I've never done a database using states where I
did not, sooner or later, need the state abbreviation, as well as other
information. And, just for the record, my much-used and
much-copied-from-database-to-database "state" lookup table is actually a
table of "US states and Canadian provinces" and, any day now, I may have a
client who will need Mexican states and their abbreviations, too.

In any case, since I need them anyway, I index on the state/province
abbreviation which may give a very minute performance advantage -- it
certainly will not be very significant.

Larry Linson
Microsoft Access MVP

 

[Douglas J Steele]

Just to play devil's advocate, at least two of the official provincial
abbreviations have changed in Canada in recent memory (Quebec used to be PQ,
and now is QC, Newfoundland and Labrador used to be NF, and now is NL). We
also got a 3rd territory a few years back, but an addition to the table
isn't as bad as a change to the PK.

 

[Guest]

States! Do you know that:

the state of North Dakota tried to change its name to just Dakota a couple
years ago?

in the '70s there was a movement to split California into three states?

West Virginia was part of Virginia until the Civil War?

there's been attempts to make Puerto Rico a state?

Just goes to show that some things considered rock solid could change in the
future. That's why I like autonumbers for primary keys.

 

[Albert D.Kallal]

State Capitol Date Admitted

Alabama Montgomery December 14, 1819
Alaska Juneau January 3, 1959
Arizona Phoenix February 14, 1912

Since the state names are unique, is there any reason not to make that
field the primary key?

Yes, there are several good reasons. First, you might find some spelling
errors. You might come out with a French version. Or, someone wants the
names to be spelled in German, or whatever tickles your fancy. If you use a
autonumber, and then start using a description for the State in place of the
name, then your database can continue to function without modification.

As others mentioned, there is much philosophy and strong views on each side
of the camp (natural keys vs autonumber keys).

My view is that when you relate a table, I simply want the database to

please give me a relation between those two tables I specify. At that
point, I give NOT one hoot about what field is used, and in fact I don't
even want to waste my brain power coming up with a field to create the
relaton. I want a one to many relaton. What you do after that is your
business!!

Here is my rant on this subject. It also explains why you don't every want
to expose the autonumber to the end user.

Be forewarned...this is a old post..and is a rant..but, it gives you the
idea of how much fervor can go into the subject...

----------------

Why would you EVE"R care what id ms-access uses for the relation?

Do you care what memory segment word gets loaded into? Do you
care if it is memory segment 32, or 8192?

Are you now to ask users with a prompt as to what memory locaton that your
word document will load into? Who cares..that junk is for comptuers to deal
with...not humans...


Who cares about a number you, and your users will NEVER see?

An autonumber is some mechanistic to generate a number. To you and me, all
we care about is that we have a relation from customers to customers invoice
table. Do we really care, or have to know what number is used?

Really, when word loads into memory, we don't care about the number used for
the memory location. Really, when ms-access has a relation between customers
and the invoice file...again we don't give a hoot about what number is used.
Me, or you never sees the segment number when word loads, and we as users
will never see the autonumber either.

These numbers are NOT for humans to see.....

There is a ZILLION kinds of internal numbers that your computer uses all day
to function. Why do you care what memory segment numbers the computer used
to load ms-access, or ms-word? Why care?

If your folks can see, or use those autonumbers...then that is your problem
with autonumbers.

The real wrong being done here is that users can see, or use the
autonumbers. I mean, do you want ms-word to start showing you the memory
segment numbers it uses to load a document into memory? It would be crazy to
force users to deal with memory segment numbers when using word.

With ms-access, YOU NOW are the software developer. So, just like those
developers who creased word, they don't show users what memory location the
documents load into. You as a developer has a responsibly to NOT LET USERS
see the autonumber.

If you need some number for your users, then you need to write your own
custom code that generates those numbers for human consumption (say, things
like invoice number etc.). You do NOT want to use the invoice number for
relations etc (you still use a internal autonumber, and that way you don't
even care if the invoice has a invoice number, or perhaps you wait a
specified time until a invoice number is given. Either way, you can still
have your relational database function...but behind the scenes it is using a
autonumber).

Your database should not crap out just because you don't have a order number
handy. Who even cares if you enter a order number, or not? Why should your
database stop function if you don't enter a order number? Even if you change
the order number, again..why should your database not work? Maybe you need
to delete the order number? (again, what on planet earth does deleing some
arbitrary number like the order number HAVE ANYTHING to do with building a
functional relation between two tables? How possibility does these two
separate concepts have anything in common?).

You users should NEVER EVER see a autonumber.

You mistake here is to try and let humans see, or even refer to, or use the
autonumber in any way. Autonumbers are NOT to be given meaning by
humans...but ONLY to your software.

Who cars if you have a order number, or not? What does the fact of having a
order number have to do with your database to functionally correctly? If you
want to require that a order number HAS to be entered, then make the order
number a required field, but that simple stupid order number HAS NOTING to
do with setting up a relation between two tables.

Setup your relation between tables with internal numbers, and your database
will JUST WORK REGARDLESS of what fields, and things you decide to store as
data. Do not go and attached some number out of the blue like a stupid order
number to build relations between your tables. Can you imagine if products
like QuickBooks, or even products like ms-word exposed internal numbers used
for relations and other internal numbers as to how the software will
function? Software uses ZILLIONS AND ZILLIONS of internal numbers and
pointers to function.

Now that YOU ARE the software developer, it is up to you to hide these
numbers. You can expose these internal numbers (like autonumbers), but that
is just rude, and just services to torture your users. Hide all the internal
number stuff....every other developer before you did this....

Why expose users to the exhaust pipe of a car when all they want to do is
drive? Software is a machine you build. Build it...make it work, and then
give it to your users. Users do NOT need to know about the kinds of teeth
used in the gears for the car...

So, the two concepts of how relations works is that many of us just believe
that setting up a relation between two tables is a conceptual idea, and HAS
NOTING to do with the data that you need to store. Others would disagree on
this concept...

By the way, there are some STRONG augments for using natural keys. For
example, if I adopt a natural key in my data, then can freely move it
between TWO DIFFERENT systems that respect this approach. (of course, you
have to have those two systems respect that approach!!).

 

[Lyle Fairfield]

Most of the reference books recommend autonum primary keys, but the
Access help says that any unique keys will work.

What are the tradeoffs?

I have several tables that have unique fields. Can I use them as
primary keys or should I define an autonum primary key?

One table has information about the 50 states in the US. The table
looks like this:

State Capitol Date Admitted
Alabama Montgomery December 14, 1819
Alaska Juneau January 3, 1959
Arizona Phoenix February 14, 1912

Since the state names are unique, is there any reason not to make that
field the primary key?

Another table has to do with grade school multiplication tables. This
is a simple table something like this:

Factors Answer
1x1 1
2x1 2
2x2 4
3x1 3
3x2 6
3x3 9
...
12x1 12
12x2 24
...
12x12 144

Here again, the Factors field (a text field) is unique. Is there any
reason not to use it as the primary key?

In both cases, the tables are small, so adding another 4-byte field is
no big deal, but I'd like to keep the tables as simple as possible
unless there is some downside.

Thanks

You could use whatever you want. IMO most of the criticism around this
topic involves autonumbers being used as the primary key, and in
addition, an attempt is made to use these autonumbers as ordinals,
perhaps sequential invoice numbers, rather than merely nominals,
identifiers of the records.
Many developers routinely create an autonumber ID in every table as
they create it. This, IMO, simplifies relationships (they are always
(ID, ID) where ID = ID), and ensures that a unique identifier exists
for each record, without concern for any meaning, duplication or
possible nullability of that identifier (Access forms often are not
updateable unless such an identifier exists).
But if one has the concepts and skill, other primary keys are fine. Of
course, many may not have the skill, and those who do will often choose
autonumbers to standardize their approach to this matter.
I use autonumbers. There are sufficient things to be planned and
decided about db design without including ... what will my primary keys
look like.

BTW, some think of Primary Key as something "special". A primary key is
simply the first created non-nullable unique index. Designating an
index as primary will move it to position one (or return an error). We
could easily do away with this term; I worked with indexes ( a thousand
times more powerful and useful than JET or SQL-Server indexes) for many
many years in the X-Base world without ever hearing it and I find no
particular value in its availability.

 

[LurfysMa]

Just to play devil's advocate, at least two of the official provincial
abbreviations have changed in Canada in recent memory (Quebec used to be PQ,
and now is QC, Newfoundland and Labrador used to be NF, and now is NL). We
also got a 3rd territory a few years back, but an addition to the table
isn't as bad as a change to the PK.

OK, OK. I'm sold. I'll use an autonum field as the primary key. I
suppose the phonetic English movement could still gather steam and
California would become Kaliforia or something. ;-)

 

[rkc]

Most of the reference books recommend autonum primary keys, but the
Access help says that any unique keys will work.

What are the tradeoffs?

The tradeoff is that some people will think you're an
idiot if you use them and some people will think you're
an idiot if you don't.

As long as you understand that adding an autonumber
as a primary key has nothing to do with the normalization
process I think they are just fine.

 

[Tony Toews]

Just be aware - for some this isn't just a matter of preference, it's
religion to a degree thats close to fanatism.

<chuckle>

Tony
--
Tony Toews, Microsoft Access MVP
Please respond only in the newsgroups so that others can
read the entire thread of messages.
Microsoft Access Links, Hints, Tips & Accounting Systems at
http://www.granite.ab.ca/accsmstr.htm

 

[hbinc]

Hi LurfysMa

Nice discussions.
Every record that has an relation with other records in other tables,
must have an unique identification, that used in all the relations. As
long as the "meaning" of the record stays the same, this identification
stays the same. Whether it is Autonumbering or Random or whatever is
not important, as long as it is unique.

Independant of the the identification is what you choose as Primary
Key. This may be your unique identification, but in fact can be any
combination of any fields, as long as they do not contain null-values.

But if you use a non-Autonumbering Primary Key, be sure that you use
your unique identification in your relations!

HBInc.

 

[onedaywhen]

Most of the reference books recommend autonum primary keys, but the
Access help says that any unique keys will work.

What are the tradeoffs?

That is a good question.

He's the position, as I see it, in brief.

Codd introduced the idea of a primary key. He later realised that all
keys are valid and that he was previously thinking non-relationally
when he assumed one key would need to be nominated as 'primary'.

RM theory has since moved on from the concept of primary keys. It was
too late for SQL, though: SQL vendors implemented primary keys,
assuming the PK would be given special meaning, and the concept of PKs
was retro-fitted to the SQL standards.

You can replace all your PRIMARY KEY constraints with NOT NULL UNIQUE
because they logically equivalent. This is what the Access help means
as referred to by the OP. However, in terms of physical SQL
implementation, PRIMARY KEY has been given special meaning. This is why
you are (correctly) still urged to designate a PRIMARY KEY for all your
tables.

What few people tell you is *how* to choose the PK.

What it comes down to is this: for Access/Jet, what does PRIMARY KEY
give you that NOT NULL UNIQUE does not? What is the special meaning for
the particular product, Access/Jet?

The answer, for Access/Jet the PK determines the (non-maintained)
clustered index, the physical ordering on disk.

So the next question is: what makes the best clustered index? The
answer to this is that a clustered index favours BETWEEN clauses and
GROUP BY clauses in SQL DML (queries, etc). In other words, your choice
of PK in SQL DDL (design) is driven by you SQL DML (queries). The
paradox here is that you can't write SQL DML before you've written your
SQL DDL, so you need to keep your PK's under review.

If you've understood the above you should come to the conclusion that a
sole autonumber column will never make a good PRIMARY KEY in
Access/Jet, because a random/incrementing integer/GUID does not make a
good clustered index. I'd suggest that anyone who uses their autonumber
column in a BETWEEN or GROUP BY construct has got something wrong in
design and/or queries. I'd further suggest that anyone who uses BETWEEN
or GROUP BY constructs which do not include columns that comprise their
PKs are likely to have made a poor choice of PK.

Jamie.

--

 

[Lyle Fairfield]

The answer, for Access/Jet the PK determines the (non-maintained)
clustered index, the physical ordering on disk.

Can you verify this?

 

[Lyle Fairfield]

What it comes down to is this: for Access/Jet, what does PRIMARY KEY
give you that NOT NULL UNIQUE does not? What is the special meaning for
the particular product, Access/Jet?

The answer, for Access/Jet the PK determines the (non-maintained)
clustered index, the physical ordering on disk.

From

http://msdn2.microsoft.com/en-us/library/wd9d69b1.aspx

THE CLUSTERED PROPERTY IS IGNORED FOR DATABASES THAT USE THE MICROSOFT
JET DATABASE ENGINE BECAUSE THE JET DATABASE ENGINE DOES NOT SUPPORT
CLUSTERED INDEXES.