Separate PK in Jxn Tbl?

[David Cressey]

Personally, I don't take the natural keys out either, so they can still
be used for the deletion.

There are really two issues being discussed in a single discussion here.

The issue of synthetic keys versus natural keys is one issue. The issue of
a composite PK in a junction table, made up of FKs, versus a new simple key
is a separable issue.

Please note that, if the two FKs under discussion both reference synthetic
PKs, all of your arguments concerning the problems of dealing with natural
keys become moot.

If we have three tables, Students, Courses, and Enrollments, where
enrollments is a junction between Students and Courses, we could have a
synthetic key, StudentID for students, and a synthetic key, CourseID, for
Courses.

The question then remains which is simpler. To define enrollments with a
composite key
(StudentID, CourseID), or to define a new synthetic key, EnrollmentID.
Neither of these two solutions uses natural keys.

I prefer to se natural keys whenever possible, but I use synthetic keys
when natural ones just won't do. When do natural keys fail to do the job?
When the poeple who control them are mismanaging them.

 

[David Cressey]

Well, that's just dumb. Checks in code can reduce database round-trips, and
therefore can improve performance, but are not and cannot be a substitute
for constraints on the tables. It is the constraints on the tables that
keeps garbage out of the database.

The idea of keeping garbage out of the database takes on an entirely
different meaning if you are dealing with hundreds of programs written in
COBOL, Java, or anything in between accessing a single Oracle database on
the one hand. On the other hand, if you are a developer creating a self
contained MS Access database cum application (tables, queries, forms,
reports, modules, etc.) all in one file, the same issues arise, but they
are resolved quite differently.

I'm not saying either one is "right" or "wrong". I'm just suggesting why an
objection that makes perfect sense to you and me might be lost on the MS
Access community.

 

[Rick Brandt]

The idea of keeping garbage out of the database takes on an entirely
different meaning if you are dealing with hundreds of programs
written in COBOL, Java, or anything in between accessing a single
Oracle database on the one hand. On the other hand, if you are a
developer creating a self contained MS Access database cum
application (tables, queries, forms, reports, modules, etc.) all in
one file, the same issues arise, but they are resolved quite
differently.

I'm not saying either one is "right" or "wrong". I'm just suggesting
why an objection that makes perfect sense to you and me might be lost
on the MS Access community.

^
some of

I think if a thorough poll was done it would show that the majority of
professional Access developers (those that make their living at it) would agree
that data integrity rules should be enforced by the database engine wherever
that is possible.

The fact is that Access is a tool predominantly for *users*, not developers, and
Microsoft appears determined with each subsequent version to make that more the
case. The majority of changes make it easier to do things incorrectly because
that makes the program easier to use for people who have no idea what they are
doing. Since that group vastly outnumbers the other one can hardly argue with
their logic from a business standpoint.

 

[Brian Selzer]

Since natural keys are data that needs to be managed, this
is an unsurprising claim. I don't throw out the data I'm supposed
to be managing either. Also consider this thread is about junction
tables. Imagine what you would have left if you threw out the
natural key of a junction table: nothing! Just the capriciously
introduced surrogate key in a table by its lonesome self.

Perhaps I've just stumbled into a sanity check for table
design: every projection of a table should have a meaningful
predicate. (Do I hear JOG's ears pricking up?) In a junction
table in which a surrogate key has been introduced, what's
the meaning of the projection of the table over the s.k. column?
Oops!

I think you're jumping the gun, Marshall. Since a surrogate key value is
just a name, what a projection over the s.k. column would leave you with is
a set of names, and the only meaning that can be derived from each element
is that there is an individual that has been so named and due to domain
closure that that individual actually exists.

 

[Sylvain Lafontaine]

Your argument about the use of a DRI WITH ON UPDATE CASCADE is an

Really? I can put my hand on my heart and say I've never seen anyone
suggest it when they cross-post to comp.databases.theory. I don't doubt
it has come up, but I do doubt it is often.

Oh, the word "often" is all relative here. Of course, in a newsgroup about
Access and/or SQL-Server, you won't see it coming very often because these
two databases don't offer support for DRI updating on cyclic relationships.
What you are more likely to see would be people complaining about the
absence of such support in SQL-Server. However, I suppose that many of
those peoples who are making this complaint have probably the use of natural
keys behind their mind (the other strong possibility beeing the use of some
form of replication); so this close the circle. I've also suppose that on
other newsgroups dedicated to systems like Oracle, the discussion about this
point is probably much more heated.

I should also add that if I remember correctly, this one was a feature that
has been promised to be released with SQL-Server 2000 but that it didn't
make the last cut and a lot of people were angry about that. Nowadays, in
regard to the next release of SQL-Server 2008, peoples seem to be more
interested with features such as the integration with .NET and other big
stuff like that than to know if Katmai will offer support for this or not.

First of all, updates to keys should be very rare. A fundamental property
of a well-chosen key is that it is reasonably stable. Imposing the little
self-discipline required to make sure you choose stable keys is
inconsequential when compared to the overall database design effort.

This is the big problem with natural keys. When the theory was first
elaborated, the point was that a natural key never change its value. That
was the point that was making the choice of a natural key on par with the
use of a surrogate key as the primary key of a table. However, like anyone
have discovered with experience, a natural key can change its value under a
set of various circonstances. One could argue that if a key can change its
value, than it's not a natural key but as you know, this argument bring
nothing in regard to help you choosing a natural key.

Now, how it's important the fact that the update to a key should be very
rare? To me, there is a big difference between 0 and 1 time but there is
not between 1 and 1 million times. If you have to put code in order to take
into account the fact that the key can change its value, it should be the
same code if the key change its value one single time for the life of the
database or multiple times each day. In many cases, you are even placed in
the situation where there is a strong possibility that the key will never
change its value a single time for the whole lifetime of the database but
that you must take into account the possibility that it might do so.

Of course, there are many possible solutions: put a lot of DRI/triggers and
other pieces of code; forbid any change (and make the clients unhappy on
many occasions); suggest to delete the records and recreate them from
scratch or even rebuild the whole database; etc.; etc. but why bother with
all these in the first place?

To me, the use of natural keys is like someone bringing me a box full of
Damocles' swords: he would usually tell me to be very careful when I will
put these on the ceiling. Later, when he will come back and see the empty
ceiling and ask me why, I would answer him that I've put the box in the
garbage bin but if he want them, he can bring the box home for free; at the
condition that I'll never see them again. The fact that some theorists like
these swords too is of no interest to me. If they like them, they too can
bring them home for free but at the same condition, that I'll never them
again.

 

[Sylvain Lafontaine]

Most Vociferous Person (MVP)? Well, I suppose that this was true then you
would have earning this little award a long time ago. As for the rest of
your post, you know the old proverb: thefts have the best locks.

--
Sylvain Lafontaine, ing.
MVP - Technologies Virtual-PC
E-mail: sylvain aei ca (fill the blanks, no spam please)

Hi Sylvain,

First, let me thank you for being so kind as to volunteer the information
that you are a Most Vociferous Person (MVP). It does a fair service to the
world when the self-aggrandizing ignorants self-declare that information.

To that, I would add that the increased simplicity of using a surrogate
(or artificial or autonumber) key as the primary key in place of a
composite key is only half their advantage.

At this point, a prudent man would Plonk! you while mentally citing Date's
_Principle of Incoherence_. Never the prudent man, instead, I observe the
absurdity of your suggestion that adding features, structures or
attributes increases simplicity. What nonsense!

The biggest problem that I have with composite keys is that they share
the same fundamental problem as natural keys: using them as the primary
key is allowing the fact that a primary key can change its value over
time. IMHO, a primary key should never be allowed to change its value
once it has been created; a assumption which will forbid the use of a
composite key in many cases.

I find your absolutism foolish suggesting ignorance and/or stupidity.

The design criteria for keys are: uniqueness, irreducibility, simplicity,
stability and familiarity (in no particular order). If any criterion is
absolute, it is uniqueness not stability.


(Of course, if you don't mind to see a primary key changing its

value after its creation then you are not concerned by this argument.).

This is not only a theoritical argument as many interfaces - like
Access - won't like to see a primary key that could change it value.

It is not a theoretical argument at all. You simply regurgitate ignorance
and stupidity.

[remaining nonsense snipped]

Plonk!

 

[David W. Fenton]

(e-mail address removed) wrote in
m:

If the users only access the tables through forms, conforming to
best practices in Access, how are they going to get garbage into
the tables?

What if there's more than one application built on top of the
database?

I, too, agree that one should put as much of the data logic in the
back end as possible.

However, that doesn't mean I use natural keys very often. I'm
definitely opposed to compound keys for any table whose PK will be a
foreign key in another table. It causes myriad problems of all sorts
(been there, done that), and despite its being theoretically
correct, just doesn't work well in practice.

Just consider one scenario:

You need to build criteria for a query-by-form interface. That means
that to query on the PK of a table with a compound PK, you end up
needing to have multiple fields in your WHERE clause. And if you're
querying multiple records in the table with the compound PK, you'll
need a complex nested OR in your WHERE clause.

I know perfectly well that theoretically speaking you're not
supposed to let your application drive the design of your schema,
but this is a case where common sense tells me that following theory
leads to enormously difficult application logic problems.

Natural keys are great for tables with a single-column natural PK.

Otherwise, surrogate keys make building an application substantially
easier.

And, BTW, I would, of course, advocate that any natural key that is
not used as the PK should naturally have a unique index on it.

And any natural key that can't have a unique index (because some
fields need to be Null) was never a candidate for PK in the first
place, and would have to have had uniqueness enforced in the
application in some fashion anyway.

 

[David W. Fenton]

m:

I certainly don't think developers should excuse sloppy RDBMS
design just because they are using access (and of course I'm sure
many of the professionals here wouldn't dream of doing so, despite
others laxness).

What *are* you talking about?

Any mistakes in schema design that you can make in Access, you can
make in any other RDBMS.

I would agree that there are many places that the wizards in Access
and the sample databases encourage sub-optimal practices. But most
professional developers aren't using either wizards or the sample
databases as their models for developing their own applications.

If your impression of Access comes from futzing with it for 10
minutes and from encountering kludged-together apps created by your
company receptionist, then you just haven't a clue what Access
offers, either as an application development platform or as a data
store (using its native Jet engine).

This is a schema question, and that is orthogonal to Access, because
Access is an application development platform. If you use Jet for
your data store, then Jet is relevant to the discussion. But you can
use any data store that offers an ISAM or ADO or ODBC drivers, and
that means all your schema issues are completely divorced from
Access itself.

That you can't seem to keep this distinction clear in your mentions
of Access demonstrates pretty clearly that you are completely
clueless about Access and really aren't in any position to be making
disparaging comments about it.

 

[David W. Fenton]

m:

No probs, although off the top of my head its gonna be a bit
contrived. With an artificial key:

Marriages {id, husband, wife, date}
Kids_from_Marriage {from_id, name, birth}

A query that asks "fetch me all the children whose mother is x"
obviously requires an equijoin, matching Marriages.id and
Kids.from_id. However with the original natural keys:

Marriages {id, husband, wife, date}
Kids_from_Marriage {mother, father, name, birth}

The same query is a simple select. That certainly seems a lot less
complicated to me

Assuming you've got some form of CASCADE UPDATE on your enforced
relationship, that will work.

But it's repeating a huge amount of data, and adding a bunch more
indexes to keep updated. Perhaps these are insignificant issues to
*you* and *your* apps, but my clients' apps (some using Jet, some
using SQL Server, some using MySQL) don't perform so well when you
add in all the overhead.

And it all leaves aside the question of how you know that
husband/wife/date is always going to be unique. I think that on any
given day in the US, there are plenty of marriages in which those
three values will be identical. You could add place. But then, in
large cities, that might not be enough. So use Postal Code in place
of place, and that might do the trick, although in large cities that
might not do it, either.

Given that I can foresee a reasonable possibility of a collision on
this candidate key as currently defined, I'd think long and hard on
whether to use it or not.

And it's one of the main problems any time you're storing data about
people in a data table -- you often lack pieces of the information,
and you run a high risk of collisions between people with the same
names.

And that fact of the real-world entities being modelled makes
finding a natural key that will work as a PK a very hard task.

 

[David W. Fenton]

:

Since natural keys are data that needs to be managed, this
is an unsurprising claim. I don't throw out the data I'm supposed
to be managing either. Also consider this thread is about junction
tables. Imagine what you would have left if you threw out the
natural key of a junction table: nothing! Just the capriciously
introduced surrogate key in a table by its lonesome self.

Perhaps I've just stumbled into a sanity check for table
design: every projection of a table should have a meaningful
predicate. (Do I hear JOG's ears pricking up?) In a junction
table in which a surrogate key has been introduced, what's
the meaning of the projection of the table over the s.k. column?
Oops!

Junction tables that:

1. include nothing but the foreign keys of the tables they are
joining,

AND

2. have no child tables

need not surrogate key.

Since I use surrogate keys in most of my schemas, most junction
tables are two-columns, and those two columns are the PK.

Once I did have a schema where there was a child table to the
junction table, and in that case, I wish I'd had a surrogate key (it
was a pure natural key design, actually, and the worst project I've
ever worked on; no one to blame but myself, as I designed the schema
from scratch), as it made it awfully difficult to work with that
table that was a child of the junction table.

 

[David W. Fenton]

m:

I do know that the debate over relationship representations has
been going on for years, neither side giving any ground. Some
aspects of it are never addressed. For example, making the two
foreign keys in a junction table a composite PRIMARY key prohibits
NULL values from either key, but declaring a separate surrogate
primary key together with a UNIQUE constraint on the
two-foreign-key-composite-key does allow NULL values for either
(or both!). This can be useful for representing unrelated entity
tuples, either childless parents or orphans.

Er, why go to all that trouble? You can represent the same childless
parents by simply omitting a record in the junction table.

 

[rkc]

Right. And then you need to change a constraint, and you modify the
code and you remember to update three out of the four forms that
implement the check, and you get garbage in the database. Or
any of a number of other things that can go wrong.

Yes. Because Access developers have no need to separate data access
from user interface from business rules. Just bind a form to a
recordsource, sprinkle some VBA code in a few events and run with it.

 

[Sylvain Lafontaine]

« But many of them seem to write as if contents as determined by address ».

The content is not determined by the address and in fact, for those who are
using surrogate keys, the exact value of an address inside the database has
zero importance. To me, a table is a collection of jars, each jar has a
serial number and the information that must be stored into the database -
the information for which the database has been create - is put inside these
jars. Each jar has its own serial number; however, the exact value of this
serial number has not importance at all in regard to what is put inside the
jar and if copy the information from this database to another database, I
don't care to see the information to be put back in a jar with the exact
same serial number.

However, proponents of the use of natural keys insist to have a strong
relationship between the serial number on the jar and what's in it. To me,
such a relationship is pointless and I feel no necessity to use it.

 

[Bob Badour]

m:




What *are* you talking about?

Any mistakes in schema design that you can make in Access, you can
make in any other RDBMS.

I would agree that there are many places that the wizards in Access
and the sample databases encourage sub-optimal practices. But most
professional developers aren't using either wizards or the sample
databases as their models for developing their own applications.

If your impression of Access comes from futzing with it for 10
minutes and from encountering kludged-together apps created by your
company receptionist, then you just haven't a clue what Access
offers, either as an application development platform or as a data
store (using its native Jet engine).

This is a schema question, and that is orthogonal to Access, because
Access is an application development platform. If you use Jet for
your data store, then Jet is relevant to the discussion. But you can
use any data store that offers an ISAM or ADO or ODBC drivers, and
that means all your schema issues are completely divorced from
Access itself.

That you can't seem to keep this distinction clear in your mentions
of Access demonstrates pretty clearly that you are completely
clueless about Access and really aren't in any position to be making
disparaging comments about it.

Nothing you have written contradicts JOG. In fact, you have agreed with
him albeit vehemently and derogatively.

 

[Sylvain Lafontaine]

This remind me of the discussion twenty five years ago between relational
databases (like SQL-Server) and non-relational databases (like ISAM
databases, dBase, etc.). It has always been right to say that for simple
queries, non-relational databases are faster than relational databases.
However, nowadays, non-relational databases have (practically) vanished
because of their inherent slowness when the queries become more and more
complicated.

This is exactly the same situation with the possibility of accelerating a
query by using a natural key: you are accelerating simple queries that are
already light and fast but on the opposite side, you are slowing down
complexe queries that are already big and slow. Not sure if going this way
is really advantageous.

--
Sylvain Lafontaine, ing.
MVP - Technologies Virtual-PC
E-mail: sylvain aei ca (fill the blanks, no spam please)

Well, that's just dumb. Checks in code can reduce database round-trips,
and
therefore can improve performance, but are not and cannot be a substitute
for constraints on the tables. It is the constraints on the tables that
keeps garbage out of the database.

If the users only access the tables through forms, conforming to best
practices in Access, how are they going to get garbage into the
tables? Now if you're trying to keep Jamie and his Excel SQL out of
your database, that's another story .
<<<<<

There can be several forms that access the same table, so you would have
to duplicate the code behind each form that accesses a table, or you can
get garbage into the database.

An extra join may be needed if the natural key from the parent table is
used
in a restrict clause. If all you have is the artificial key from the
parent
table, then you have to join in order to access the natural key columns.
With natural keys, the natural key values from the parent table also
appear
in the child table, so there isn't any need to join. Bottom line: joins
of
artificial keys are typically faster than joins of natural keys due to
the
size of the comparands, but with natural keys, fewer joins may be
needed..

If you're planning on using a natural key column in the child table as
part of a join then doesn't it make sense to include that field in the
child table?

Still waiting...


A typical schema with artificial keys:

Customer {CustomerKey, CustomerNo, ...}
Key {CustomerKey}, Key {CustomerNo}

Item {ItemKey, ItemNo, ...}
Key {ItemKey}, Key {ItemNo}

CI {CustomerItemKey, CustomerKey, ItemKey, CustomerItemNo}
Key {CustomerItemKey}, Key {CustomerKey, ItemKey}
CI[ItemKey] IN Item[ItemKey]
CI[CustomerKey] IN Customer[CustomerKey]

SOLine {SOLineKey, SOKey, SOLineNo, CustomerItemKey, Quantity, Price}
Key {SOLineKey}, Key {SOKey, SOLineNo}
SOLine[CustomerItemKey] IN CI[CustomerItemKey]


A typical schema with natural keys

Customer {CustomerNo, ...}
Key {CustomerNo}

Item {ItemNo, ...}
Key {ItemNo}

CI {CustomerNo, ItemNo, CustomerItemNo}
KEY {CustomerNo, ItemNo}
CI[CustomerNo] IN Customer[CustomerNo]
CI[ItemNo] IN Item[ItemNo]

SOLine {SO#, SOLineNo, CustomerNo, ItemNo, Quantity, Price}
SOLine[CustomerNo, ItemNo] IN CI[CustomerNo, ItemNo]


Now write a query that returns how many of item '12345' were sold to
customer '4321'

It should be obvious that with the natural keys, no joins are
necessary--it's just a simple select from SOLine since all of the
information is actually /in/ SOLine; whereas, with the artifical keys,
several joins are required because in order to query by item number and
customer number, SOLine must be joined to CI which must then be joined to
Customer and Item.

 

[Bob Badour]

(e-mail address removed) wrote in
m:


What if there's more than one application built on top of the
database?

I, too, agree that one should put as much of the data logic in the
back end as possible.

However, that doesn't mean I use natural keys very often. I'm
definitely opposed to compound keys for any table whose PK will be a
foreign key in another table. It causes myriad problems of all sorts
(been there, done that), and despite its being theoretically
correct, just doesn't work well in practice.

Reference is one issue that increases the tradeoff importance of
simplicity relative to the other design criteria.

Just consider one scenario:

You need to build criteria for a query-by-form interface. That means
that to query on the PK of a table with a compound PK, you end up
needing to have multiple fields in your WHERE clause. And if you're
querying multiple records in the table with the compound PK, you'll
need a complex nested OR in your WHERE clause.

I know perfectly well that theoretically speaking you're not
supposed to let your application drive the design of your schema,
but this is a case where common sense tells me that following theory
leads to enormously difficult application logic problems.

I have yet to see any evidence from you to suggest you know the first
thing about theory. Perhaps you should strive to learn a little more
about it before blathering on about it.

Natural keys are great for tables with a single-column natural PK.

A natural key is neither more nor less than a familiar surrogate.

Otherwise, surrogate keys make building an application substantially
easier.

I disagree. Since all keys are fundamentally the same thing, surrogacy
is irrelevant. The design criteria for keys are: uniqueness,
irreducibility, simplicity, stability and familiarity (in no particular
order.)

And, BTW, I would, of course, advocate that any natural key that is
not used as the PK should naturally have a unique index on it.

You confuse physical and logical issues. One should declare all logical
constraints regardless of the indexes used.

And any natural key that can't have a unique index (because some
fields need to be Null) was never a candidate for PK in the first
place, and would have to have had uniqueness enforced in the
application in some fashion anyway.

One would have to be an idiot to design anything allowing NULL in the
first place.

 

[James A. Fortune]

The idea of keeping garbage out of the database takes on an entirely
different meaning if you are dealing with hundreds of programs written in
COBOL, Java, or anything in between accessing a single Oracle database on
the one hand. On the other hand, if you are a developer creating a self
contained MS Access database cum application (tables, queries, forms,
reports, modules, etc.) all in one file, the same issues arise, but they
are resolved quite differently.

I'm not saying either one is "right" or "wrong". I'm just suggesting why an
objection that makes perfect sense to you and me might be lost on the MS
Access community.

I am only speaking for myself. I may be the only Access programmer on
the planet who validates input the way I do in code. I am not unaware
of the desirability of enforcing constraints at the table level. My
reluctance to depend on error trapping is shared by very few, if any,
Access developers. Those who judge my unorthodox style choices as
ignorant or incorrect presume much.

For example, in "Joe Celko's SQL PROGRAMMING STYLE:

Q: Couldn't a natural compound key become very long?

A1: So what? This is the 21st century, and we have much better
computers than we did in the 1950s when key size was a real physical
issue. What is funny to me is the number of idiots who replace a
natural two- or three- integer compound key with a huge GUID, which no
human being or other system can possibly understand, because they think
it will be faster and easy to program.

....

The auto-numbering features are a holdover from the early SQLs, which
were based on contiguous storage file systems. The data was kept in
physically contiguous disk pages, in physically contiguous rows, made up
of physically contiguous columns. In short, just like a deck of
punchcards or a magnetic tape. Most programmers still carry that mental
model, too.

The most pathetic part is that many here even parrot his (incorrect)
reasoning about my reasoning. I submit that they, not I, are succumbing
to subtle psychological effects.

James A. Fortune
(e-mail address removed)

 

[David Cressey]

This remind me of the discussion twenty five years ago between relational
databases (like SQL-Server) and non-relational databases (like ISAM
databases, dBase, etc.). It has always been right to say that for simple
queries, non-relational databases are faster than relational databases.
However, nowadays, non-relational databases have (practically) vanished
because of their inherent slowness when the queries become more and more
complicated.

I don't believe the above is correct. It's still the case that a graph
based DBMS
like IMS can otperform an SQL based DBMS for queries that were anticipated
at the time the graph was designed. This is not a function of the
complexity of the query. It's a function of the relationship between the
path the query must take through the data and the paths that are made
blindingly fast by the graph set up at stata store time.

Essentially, an index gives and SQL engine exactly what it needs to locate
table rows: pointers to table rows.

Where the SQL DBMS (and the databases built using it) are superior to a
graph based DBMS (and the databases built using it) is flexibility. When
an unanticipated query occurs in a graph based system, the DBMS is reduced
to doing a brute force search (if it can even do that much). Either that or
the database designer must redesign the database, unload and reload all the
data, and revise any application programs that were dependent on the old
structure. That's usually prohibitively expensive.

By contrast the SQL DBMS can generally build an in memory index when needed,
or a new index can be built by the database designer without unloading any
data or requiring any revisions to programs.

This is exactly the same situation with the possibility of accelerating a
query by using a natural key: you are accelerating simple queries that are
already light and fast but on the opposite side, you are slowing down
complexe queries that are already big and slow. Not sure if going this way
is really advantageous.

I disagree.

--
Sylvain Lafontaine, ing.
MVP - Technologies Virtual-PC
E-mail: sylvain aei ca (fill the blanks, no spam please)

"James A. Fortune" <[email protected]> wrote in messagenews:%

Access programmers use forms to interact with the data. If I follow
Jamie's advice and constrain the data at both the table level and in
code,
then your points make more sense. Right now, they're just arguments for
me not to constrain the data at the table level because the reasons you
gave might make natural keys preferable in that situation .

Well, that's just dumb. Checks in code can reduce database round-trips,
and
therefore can improve performance, but are not and cannot be a substitute
for constraints on the tables. It is the constraints on the tables that
keeps garbage out of the database.

If the users only access the tables through forms, conforming to best
practices in Access, how are they going to get garbage into the
tables? Now if you're trying to keep Jamie and his Excel SQL out of
your database, that's another story .
<<<<<

There can be several forms that access the same table, so you would have
to duplicate the code behind each form that accesses a table, or you can
get garbage into the database.

* Referencing an artificial key in a child table can complicates
queries - and not just with a longer restrict clause, but with a whole
extra join that may well have been unrequired if a natural key had
been used.

I don't agree with that point. The child table can contain the
AutoNumber
primary key from the main table as a foreign key if desired. I don't
see
how using the natural key fields requires less joins than that. Maybe
an
example would help me understand what you mean.

An extra join may be needed if the natural key from the parent table is
used
in a restrict clause. If all you have is the artificial key from the
parent
table, then you have to join in order to access the natural key columns.
With natural keys, the natural key values from the parent table also
appear
in the child table, so there isn't any need to join. Bottom line: joins
of
artificial keys are typically faster than joins of natural keys due to
the
size of the comparands, but with natural keys, fewer joins may be
needed..

If you're planning on using a natural key column in the child table as
part of a join then doesn't it make sense to include that field in the
child table?

Still waiting...


A typical schema with artificial keys:

Customer {CustomerKey, CustomerNo, ...}
Key {CustomerKey}, Key {CustomerNo}

Item {ItemKey, ItemNo, ...}
Key {ItemKey}, Key {ItemNo}

CI {CustomerItemKey, CustomerKey, ItemKey, CustomerItemNo}
Key {CustomerItemKey}, Key {CustomerKey, ItemKey}
CI[ItemKey] IN Item[ItemKey]
CI[CustomerKey] IN Customer[CustomerKey]

SOLine {SOLineKey, SOKey, SOLineNo, CustomerItemKey, Quantity, Price}
Key {SOLineKey}, Key {SOKey, SOLineNo}
SOLine[CustomerItemKey] IN CI[CustomerItemKey]


A typical schema with natural keys

Customer {CustomerNo, ...}
Key {CustomerNo}

Item {ItemNo, ...}
Key {ItemNo}

CI {CustomerNo, ItemNo, CustomerItemNo}
KEY {CustomerNo, ItemNo}
CI[CustomerNo] IN Customer[CustomerNo]
CI[ItemNo] IN Item[ItemNo]

SOLine {SO#, SOLineNo, CustomerNo, ItemNo, Quantity, Price}
SOLine[CustomerNo, ItemNo] IN CI[CustomerNo, ItemNo]


Now write a query that returns how many of item '12345' were sold to
customer '4321'

It should be obvious that with the natural keys, no joins are
necessary--it's just a simple select from SOLine since all of the
information is actually /in/ SOLine; whereas, with the artifical keys,
several joins are required because in order to query by item number and
customer number, SOLine must be joined to CI which must then be joined to
Customer and Item.

 

[David Cressey]

Whenever I've created junction tables [sic] in the past, I always made

the PK of the junction table the combined pks from the two other tables..
<<

I wish I knew where people got the term "junction table" when they
mean a table that models a relationship . I guess if you are still
thinking in terms PK-FK being pointer chains and structure and not
relational, then you would borrow old terminology from Network
Databases. And bring that mindset with you, too.

It probably came from the term "junction record" in CODASYL databases, where
a record can be a member of two sets, in order to enable a many-to-many
connection between
the owners of the record in the two sets.

Much of the early (Bowdlerized) description of relational databases was
written with reference back to the (at the time) more familiar CODASYL
databases.

 

[Bob Badour]

I don't believe the above is correct. It's still the case that a graph
based DBMS
like IMS can otperform an SQL based DBMS for queries that were anticipated
at the time the graph was designed. This is not a function of the
complexity of the query.

I, of course, disagree. A graph based dbms has no performance advantage
over a relational dbms because a relational dbms may use exactly the
same physical structures.

[snip]