One-To-One Tables

"Steve" schreef in bericht
...
Okay, Add another table to my suggested tables ...
TblAgreementRequirement
AgreementRequirementID
AgreementID
RequirementID
RequirementMet (Yes/No)

The RequirementMet field is your validation. You don't need a validation
table.

Steve



Get lost $teve,
No-one wants you here... no-one needs you here...
OP look at http://home.tiscali.nl/arracom/whoissteve.html

Arno R

John, Jerry, Steve:

I THINK I'VE GOT IT!! It came to me last night driving home from work. I
need the Agreements table and the Requirements table in a many-to-many
relationship, with the Certificates table serving as the junction table. But
since the Requirements table will actually do double duty as providing both
insurance requirement values and certificate offering values, I will refer to
it as the Insurance Parameters table instead.

TblAgreements
AgreementID
Agreement fields

TblInsParameters
InsParameterID
InsParameterDescrip
Other fields, if any

TblCertificates
CertificateID
AgreementID
InsRequirementID (same as InsParameterID)
InsOfferingID (same as InsParameterID)

So if the InsOfferingID and the InsRequirementID match, then the Certificate
is valid for that one particular parameter.

Now, it is not clear from my reference books whether a junction table must
use a composite primary key consisting of values that match the primary keys
from the two joined tables, or, whether you are free to use a separate
primary key. I hope the latter is the case, because I've grown prejudiced
against composite keys.

I wasn't sure where in the thread to put this posting, so I put it here in
hopes you all find it. Thanks for your suggestions, and I would appreciate
any further suggestions, corrections or advice you may have to offer on this
plan of attack. I'm sure as I get on with this there will be plenty of
unforeseen obstacles to negotiate.
--OBP


"John W. Vinson" wrote:

On Tue, 25 Aug 2009 15:18:11 -0700, oldblindpew
wrote:

Thanks for your reply. See also my response to Jerry Whittle.

This is about Certificates of Insurance furnished by subcontractors. Much
of my design is driven by the standard certificate form commonly seen in the
industry. This certificate form has lots of fields on it, and in order to
validate the certificate, each field has to be checked. My approach has been
to create one table for the certificate information and another similar table
to store the results of the validation, if ever I can get there.

To check or validate the certificate, there must be requirements to check
against. Originally I had a third table consisting of three records defining
three standard sets of insurance requirements, because we took sort of a
"Small, Medum, Large" approach to our requirements. Each Agreement pointed
to one of these three standard sets of requirements. But since it is
possible for any given agreement to modify or waive any given insurance
parameter, it seems better to let each agreement have its own unique set of
insurance requirements. Thus I arrive at three tables each relating back to
the Agreements table in a one-to-one fashion.

In order to ease the task of establishing the insurance requirements for
each agreement, I'll need to figure out how to enable the user to copy a
requirements record from one of the three standards, or from an existing
agreement, and then make whatever modifications may be called for by the
Agreement.

I appreciate your taking time to consider this and offer advice on whether
it matters how the tables are linked. Or that the whole effort would be
better served by a totally different approach. --Thanks, OBP

I think it WOULD be better served by a totally differnent approach - a
normalized Access database. g

It sounds very much to me like you have a very straightforward Many to Many
relationship between Certificates and Requirements. Each certificate has many
requirements; each requirement may occur on many Certificates.

You're solving the problem using spreadsheet logic: one field (column) for
each requirement. This is not ideal in Access! If your "Small" template has
(say) seventeen requirements, then you could have seventeen *RECORDS* in a
CertRequirements table, linked to this particular subcontractor's certificate,
and to a table of all the possible requirements that might arise (seventeen of
them from seventeen different records, in this instance). This would let you
add new requirements, waive individual requirements, add modifying notes or
comments to a requirement, etc. etc.

--

John W. Vinson [MVP]

On Thu, 27 Aug 2009 07:42:06 -0700, oldblindpew
wrote:

Now, it is not clear from my reference books whether a junction table must
use a composite primary key consisting of values that match the primary keys
from the two joined tables, or, whether you are free to use a separate
primary key. I hope the latter is the case, because I've grown prejudiced
against composite keys.

You can indeed put in an autonumber primary key as a surrogate; I would
recommend putting a unique Index on the combination of the two fields to
prevent inadvertant duplicates.

Glad you got it working.
--

John W. Vinson [MVP]

Well, it isn't working yet, but at least I have a plan, thanks largely to
you. If you hadn't replied to my original question, I would probably still
be thinking of this like a spreadsheet, which might have proven functional,
but not optimal. I will pay close attention to your advice about the Index,
also. Thanks again and again!
-OBP

"John W. Vinson" wrote:

On Thu, 27 Aug 2009 07:42:06 -0700, oldblindpew
wrote:

Now, it is not clear from my reference books whether a junction table must
use a composite primary key consisting of values that match the primary keys
from the two joined tables, or, whether you are free to use a separate
primary key. I hope the latter is the case, because I've grown prejudiced
against composite keys.

You can indeed put in an autonumber primary key as a surrogate; I would
recommend putting a unique Index on the combination of the two fields to
prevent inadvertant duplicates.

Glad you got it working.
--

John W. Vinson [MVP]

=?Utf-8?B?b2xkYmxpbmRwZXc=?=
wrote in :

Now, it is not clear from my reference books whether a junction
table must use a composite primary key consisting of values that
match the primary keys from the two joined tables, or, whether
you are free to use a separate primary key. I hope the latter is
the case, because I've grown prejudiced against composite keys.

I would never use a surrogate key in a join table unless there were
tables related to the PK of the join table. For instance, if you
have a junction table between person and employer and you need to
have multiple titles for each of the records in the junction table,
then you'd need to store the junction table's PK in the storing the
position titles.

tblPerson PK PersonID
tblCompany PK CompanyID
tblPersonCompany PK PersonID+CompanyID, FK PersonID, FK
CompanyID tblTitle PK TitleID, FK PersonID+CompanyID

versus

tblPerson PK PersonID
tblCompany PK CompanyID
tblPersonCompany PK PersonCompanyID, FK PersonID, FK CompanyID
tblTitle PK TitleID, FK PersonCompanyID

But this is what I consider an unusual case. It's very seldom that
you need a many-to-one relationship beween a child table and have a
junction table be the parent record. I had to strain to come up with
such a situation, and I've encountered it only a few times in my 15+
years of database application design.

--
David W. Fenton http://www.dfenton.com/
usenet at dfenton dot com http://www.dfenton.com/DFA/

Hi, David, and thanks for your reply.

Side-issue: Person-to-Company sounds like a many-to-one relationship. I
thought the purpose of join tables was to handle many-to-many relationships.

Main issue: I like the concept of composite keys, but I read somewhere on
the internet from one published expert that composite keys should be avoided
because they create problems. My own experience is that even though Access
knows that a composite key is two fields that sometimes act as one, Access
apparently does not provide ready-made support for this reality. For
example, I like to use a list box as a navigation tool to allow the user to
pick a record, but this won't work with composite keys because the list box
is incapable of passing the value of more than one field.

Another question: In your first example, does your junction table actually
store PersonID and CompanyID twice, once as PK and again as FK?

Thanks, OBP


"David W. Fenton" wrote:

=?Utf-8?B?b2xkYmxpbmRwZXc=?=
wrote in :

Now, it is not clear from my reference books whether a junction
table must use a composite primary key consisting of values that
match the primary keys from the two joined tables, or, whether
you are free to use a separate primary key. I hope the latter is
the case, because I've grown prejudiced against composite keys.

I would never use a surrogate key in a join table unless there were
tables related to the PK of the join table. For instance, if you
have a junction table between person and employer and you need to
have multiple titles for each of the records in the junction table,
then you'd need to store the junction table's PK in the storing the
position titles.

tblPerson PK PersonID
tblCompany PK CompanyID
tblPersonCompany PK PersonID+CompanyID, FK PersonID, FK
CompanyID tblTitle PK TitleID, FK PersonID+CompanyID

versus

tblPerson PK PersonID
tblCompany PK CompanyID
tblPersonCompany PK PersonCompanyID, FK PersonID, FK CompanyID
tblTitle PK TitleID, FK PersonCompanyID

But this is what I consider an unusual case. It's very seldom that
you need a many-to-one relationship beween a child table and have a
junction table be the parent record. I had to strain to come up with
such a situation, and I've encountered it only a few times in my 15+
years of database application design.

--
David W. Fenton http://www.dfenton.com/
usenet at dfenton dot com http://www.dfenton.com/DFA/

I thought you were withdrawing from further response.

I remember seeing the posting you have copied here, but it isn't in this
thread.

"Steve" wrote:

Again, here is my response .........

Actually you don't have one-to-one relationships! You have a one-to-many
relationship between Agreements and Requirements. You have a one-to-many
relationship between Requirements and Certificates. You have a one-to-many
relationship between Requirements and Validation Checks. Your tables should
look like:
TblAgreement
AgreementID
Agreement Fields

TblRequirement
RequirementID
AgreementID
Requirement Fields

TblValidationCheck
ValidationCheckID
RequirementID
Validation Check Fields

This is just a basic outline since you gave no details about agreements,
requirements and validation checks. If you provide more detail, I will
expand upon the above.

Steve


"oldblindpew" wrote in message
...
Nope. Steve said I got three initial responses to my question, one from
Jerry, one from Steve and one from John. I see only Jerry's and John's.
--OBP

"Duane Hookom" wrote:

I think $teve was referring to an earlier thread from the same OP.

--
Duane Hookom
Microsoft Access MVP


"John W. Vinson" wrote:

On Wed, 26 Aug 2009 11:32:47 -0400, "Steve"
wrote:

You got three initial responses, Jerry, myself (Steve) and John.

I think you may have propagation problems, Steve: your response did not
make
it to my server (and probably not to oldblindpew's either).

--

John W. Vinson [MVP]

=?Utf-8?B?b2xkYmxpbmRwZXc=?=
wrote in :

Side-issue: Person-to-Company sounds like a many-to-one
relationship. I thought the purpose of join tables was to handle
many-to-many relationships.

Person-to-Company is very often a many-to-many because a person can
have a position at more than one company/organization. I could be
both president and CEO of ACME Corp., and also on the board of
InfiniCorp, and then a partner in PrivateVentures, LLC. What I've
just described is N:N for person/company, and 1:N for a person's
position(s) within each company.

Main issue: I like the concept of composite keys, but I read
somewhere on the internet from one published expert that composite
keys should be avoided because they create problems.

They do. They repeat a whole lot of data, which means you lose the
efficiency of Jet/ACE when retrieving the index pages, and then only
the needed data pages. If most of the data is in the foreign key,
Jet/ACE may not need to retrieve anything but the index, but the
number of index pages isn't going to be much larger for the same
number of rows, so I'd think that those composite keys would cause a
lot more data to be pulled across the Internet.

I don't use a composite key for any table whose PK is the foreign
key in another table. That is, if the table is not the parent in a
relationship with another table, a composite key is fine, particular
if it's a composite key of foreign keys, as in a many-to-many join
table.

My own experience is that even though Access
knows that a composite key is two fields that sometimes act as
one, Access apparently does not provide ready-made support for
this reality. For example, I like to use a list box as a
navigation tool to allow the user to pick a record, but this won't
work with composite keys because the list box is incapable of
passing the value of more than one field.

Well, not as a bound column, but you can still get the data in the
other columns. I think you use ItemData(i).Column -- I always have
to look it up, to be honest.

But for a bound listbox, it doesn't work well.

I don't bind many listboxes, but I certainly use lots of bound combo
boxes, and the problem with composite keys is identical there.

Another question: In your first example, does your junction table
actually store PersonID and CompanyID twice, once as PK and again
as FK?

No. I was describing the INDEXES not the fields. The table has only
the two fields, and there's a composite PK made up of the two
fields, and each field is individually a FK.

--
David W. Fenton http://www.dfenton.com/
usenet at dfenton dot com http://www.dfenton.com/DFA/

David,
Thank you for your clear and practical answers.

I have reexamined the question of reading data from various columns in a
list box selection. I had thought before that this could only be
accomplished for a multiselect list box, and obviously, selecting more than
one record to navigate to would be undesirable. However, it appears one can
read any column in a list box selection by using the list box's Column
property. I haven't tested it yet, though.

As for the main topic of this thread, I have about come full circle back to
my original plan of using 1:1 tables for insurance information. This is due
mostly to the nature of the data: Insurance requirements are not all the same
kind of thing, and I don't think it would do to put them all in one master
table, like state name abbreviations. Also, I have many Agreements in my
Agreements table, but not all of them have Insurance, so this is a classic
example where a separate 1:1 table is justified.

Some may argue that store inventory items are all different, but they all go
in one table. My Agreement is just an Order, and my Insurance Requirements
just the Order Detail. Maybe, but store items share a common set of fields:
Item No, Description, Quantity, Unit of Measure, Unit Price, Shipping Weight.
Insurance requirements, in contrast, can be Y/N, Dollar amounts, Dates, or
Numeric values.

I think I'll just have to go ahead and find out the hard way. One thing
about Access is that so far, with my simple application, it hasn't been too
hard to make changes.
--OBP

"David W. Fenton" wrote:

=?Utf-8?B?b2xkYmxpbmRwZXc=?=
wrote in :

Side-issue: Person-to-Company sounds like a many-to-one
relationship. I thought the purpose of join tables was to handle
many-to-many relationships.

Person-to-Company is very often a many-to-many because a person can
have a position at more than one company/organization. I could be
both president and CEO of ACME Corp., and also on the board of
InfiniCorp, and then a partner in PrivateVentures, LLC. What I've
just described is N:N for person/company, and 1:N for a person's
position(s) within each company.

Main issue: I like the concept of composite keys, but I read
somewhere on the internet from one published expert that composite
keys should be avoided because they create problems.

They do. They repeat a whole lot of data, which means you lose the
efficiency of Jet/ACE when retrieving the index pages, and then only
the needed data pages. If most of the data is in the foreign key,
Jet/ACE may not need to retrieve anything but the index, but the
number of index pages isn't going to be much larger for the same
number of rows, so I'd think that those composite keys would cause a
lot more data to be pulled across the Internet.

I don't use a composite key for any table whose PK is the foreign
key in another table. That is, if the table is not the parent in a
relationship with another table, a composite key is fine, particular
if it's a composite key of foreign keys, as in a many-to-many join
table.

My own experience is that even though Access
knows that a composite key is two fields that sometimes act as
one, Access apparently does not provide ready-made support for
this reality. For example, I like to use a list box as a
navigation tool to allow the user to pick a record, but this won't
work with composite keys because the list box is incapable of
passing the value of more than one field.

Well, not as a bound column, but you can still get the data in the
other columns. I think you use ItemData(i).Column -- I always have
to look it up, to be honest.

But for a bound listbox, it doesn't work well.

I don't bind many listboxes, but I certainly use lots of bound combo
boxes, and the problem with composite keys is identical there.

Another question: In your first example, does your junction table
actually store PersonID and CompanyID twice, once as PK and again
as FK?

No. I was describing the INDEXES not the fields. The table has only
the two fields, and there's a composite PK made up of the two
fields, and each field is individually a FK.

--
David W. Fenton http://www.dfenton.com/
usenet at dfenton dot com http://www.dfenton.com/DFA/

=?Utf-8?B?b2xkYmxpbmRwZXc=?=
wrote in :

As for the main topic of this thread, I have about come full
circle back to my original plan of using 1:1 tables for insurance
information. This is due mostly to the nature of the data:
Insurance requirements are not all the same kind of thing, and I
don't think it would do to put them all in one master table, like
state name abbreviations. Also, I have many Agreements in my
Agreements table, but not all of them have Insurance, so this is a
classic example where a separate 1:1 table is justified.

I used to be big on supertypes and subtypes, but found that the
outer joins needed to display all of them in a single list (or
UNION) were an unacceptable performance drain.

Now, with server-side views doing the joins, it might not be as bad,
but with Jet/ACE, it can be a complete killer.

That said, I just implemented a Customer History table that has
foreign keys on 3 different tables and does exactly what I just
described. It is performing just fine -- dunno why, but maybe I'm
just better at it than I used to be.

The perfect 1:1 scenario is when you create a basic record and then
some records then eventually reach a state when a bunch of other
fields need to be filled out, but not all of the basic records reach
that point. In a case management database for a psychiatric clinic,
the 1:1 side table recording the demographics, and were not filled
out until the case was closed. That was a perfect application of
1:1, as cases that never closed but were simply abandoned did not
have the demographics. It also made it easy to deal with the
demographic data as separate from the information specific to the
individual, which was good for confidentiality.

But I haven't encountered that many such situations.

--
David W. Fenton http://www.dfenton.com/
usenet at dfenton dot com http://www.dfenton.com/DFA/