Multi-Field Primary Key

I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique. Why not?
Is this a bug? I thought that is the whole purpose of doing multi-fields is
to achieve guaranteed uniqueness?

Not sure I follow the logic here. AutoNumbers are normally unique on their
own? So why would you want to use an autonumber plus another field?

If you really want to do it anyway, presumably Phone is Text field (not a
number field), so you can set these properties:
Allow Zero Length Yes
Required Yes
Default Value ""

The field will now contain a zero-length string (ZLS) instead of a Null
where this is no phone number, and so you can use it as part of the primary
key. You will also need to code so that the field is set to a ZLS when a
phone number is deleted.

--
Allen Browne - Microsoft MVP. Perth, Western Australia
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.

"nickh" wrote in message
...
I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique. Why
not?
Is this a bug? I thought that is the whole purpose of doing multi-fields
is
to achieve guaranteed uniqueness?

I use the [PHONE] portion only to link in one-to-many to another table that
contains records of activity on that [PHONE]. I am assuming that the primary
key designation causes a faster lookup during an innerjoin than just an
indexed field? (Is that assumption correct?) I.E., the [PHONE]+[AUTONUMBER]
would be in the same position in the primary key index as [PHONE] alone, but
the [PHONE]+[AUTONUMBER] achieves uniqueness whereas [PHONE] alone isn't.
Actually the problem is that a lot of records have a null [PHONE] (because we
don't have a phone], so then we need the [AUTONUMBER] portion to achieve
uniqueness. So with your ZLS the index would look like
""1
""154
""894
""954
et cetera

whereas the non-null records would have 10-digit [PHONE] + [AUTONUMBER]:
2125551212955
2135551212956
2145551212957
et cetera

Maybe my logic is just totally flawed here. I always learned in db class
that the primary key is faster than just indexing alone, so you always want
to try to create a primary key to maximize the speed of lookups and joins.



"Allen Browne" wrote:

Not sure I follow the logic here. AutoNumbers are normally unique on their
own? So why would you want to use an autonumber plus another field?

If you really want to do it anyway, presumably Phone is Text field (not a
number field), so you can set these properties:
Allow Zero Length Yes
Required Yes
Default Value ""

The field will now contain a zero-length string (ZLS) instead of a Null
where this is no phone number, and so you can use it as part of the primary
key. You will also need to code so that the field is set to a ZLS when a
phone number is deleted.

--
Allen Browne - Microsoft MVP. Perth, Western Australia
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.

"nickh" wrote in message
...
I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique. Why
not?
Is this a bug? I thought that is the whole purpose of doing multi-fields
is
to achieve guaranteed uniqueness?

On Thu, 4 Jan 2007 21:41:01 -0800, nickh
wrote:

I use the [PHONE] portion only to link in one-to-many to another table that
contains records of activity on that [PHONE]. I am assuming that the primary
key designation causes a faster lookup during an innerjoin than just an
indexed field? (Is that assumption correct?)

No, it is not.

Put an Index (unique or not as appropriate) on the Phone field,
distinct from the Primary Key. The Autonumber alone should be the PK;
the index on Phone will make your joins work just fine (better than a
composite index would do in fact).

John W. Vinson[MVP]

On Jan 5, 5:06 am, "Allen Browne" wrote:
I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique. Why
not?
Is this a bug?

Such a key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key and
'primary' has no special meaning. The UNIQUE constraint can still be
used in a FOREIGN KEY constraint (see OT note below).
Perhaps the better approach is to ask: what does PRIMARY KEY give me
that UNIQUE does not, aside from requiring that all values be not null?
(FWIW columns in the PRIMARY KEY designation can be nullable, it's just
you can't actually put the null value in them)...

The key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key.
The UNIQUE constraint can still be used in a FOREIGN KEY.

AutoNumbers are normally unique on their
own? So why would you want to use an autonumber plus another field?

A valid answer to your question is clustering or physical ordering on
disk.

The better 'phone book' example would be the desire to cluster on the
subscribers' names. The phone number (using a natural key) or
autonumber would be required to break duplicates because subscribers'
names may not be unique.

Care is required in declaring the columns in the correct order i.e.
PRIMARY KEY (name, phone_number) i.e. the columns should be in
left-to-right order of significance to the clustering.

Clustering on name would favour BETWEEN (e.g. grab all names beginning
with 'B') and GROUP BY on the name because the rows would be physically
contiguous (and on the same page) to begin with and would be faster to
fetch. Think how 'useful' a paper phone book physically order on
telephone number would be to the average user.

Clustering on phone number or *random* autonumber on the other hand
would favour concurrency but a sequential autonumber PRIMARY KEY may be
the worst choice in any case.

[OT ]

FWIW nulls in foreign key constraints is one of the few areas where the
Jet implementation is IMO better than SQL Server's (and, Allen, I know
you are a fan of the 'CASCADE to null' feature in Jet 4.0 g).

SQL Server will disregard all values in the referencing columns if one
of then contains a null and also will not cascade them e.g. from a null
value to a not null value. When you see how Jet handles it, it makes
the SQL Server approach seem very odd e.g.

CREATE TABLE Table1 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER, UNIQUE (key_col1, key_col2)
)
;
CREATE TABLE Table2 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER,
FOREIGN KEY (key_col1, key_col2)
REFERENCES Table1 (key_col1, key_col2)
ON DELETE CASCADE
ON UPDATE CASCADE
)
;

INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (3, NULL)
;

That last insert would fail in Jet but in SQL Server it would succeed
on the logic, I guess, that Table1.col2 = Table2.col2 returns unknown
therefore should not fail; Jet's apparent logic 'NULL must equal NULL'
is harder to defend 'relationally'. However, what seems strange IMO is
the SQL Server CASCADE behaviour:

UPDATE Table1
SET key_col2 = 1
WHERE key_col1 = 1
;

The above change does not cascade to the referencing table (Table2) in
SQL Server. While the inability to compare null values between tables
may be sound 'relationally' it would seem to be unintuitive and not the
desired effect. Similarly, when deleting:

DELETE
FROM Table1
WHERE key_col1 = 2

the row is removed from the referencing table in Jet but not in SQL
Server.

I think the Jet behaviour has more utility. Such 'practical' decisions
are hardly unprecedented, even my the famously strict ANSI SQL
standards committees. For example, if one null value cannot be compared
as being equal to another null value then why do they GROUP together?
e.g.

DELETE FROM Table1
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
SELECT key_col2, COUNT(*)
FROM Table1
GROUP BY key_col2
;

The answer is, at least in part, because most people would expect it to
work this way and such an approach has the greater practical utility.

Jamie.

--

It is probably more important for the Inner Join to put an Index on the
ForeignKey [frg_PHONE] in the "Many" Table in this case.

Something troubles me: Do you have Activities that don't involve [PHONE]?

--
HTH
Van T. Dinh
MVP (Access)



"nickh" wrote in message
news
I use the [PHONE] portion only to link in one-to-many to another table that
contains records of activity on that [PHONE]. I am assuming that the
primary
key designation causes a faster lookup during an innerjoin than just an
indexed field? (Is that assumption correct?) I.E., the
[PHONE]+[AUTONUMBER]
would be in the same position in the primary key index as [PHONE] alone,
but
the [PHONE]+[AUTONUMBER] achieves uniqueness whereas [PHONE] alone isn't.
Actually the problem is that a lot of records have a null [PHONE] (because
we
don't have a phone], so then we need the [AUTONUMBER] portion to achieve
uniqueness. So with your ZLS the index would look like
""1
""154
""894
""954
et cetera

whereas the non-null records would have 10-digit [PHONE] + [AUTONUMBER]:
2125551212955
2135551212956
2145551212957
et cetera

Maybe my logic is just totally flawed here. I always learned in db class
that the primary key is faster than just indexing alone, so you always
want
to try to create a primary key to maximize the speed of lookups and joins.

So, Jamie, if I am understanding your points, you are saying that there
actually is a benefit to my idea in terms of the physical fragmentation on
the disk. I.E. the worst arrangement is to have the autonumber alone be the
primary key because it is completely random and irrelevant and there will be
the most disk head movement as I try to fetch phone#'s from the table
(assuming I am fetching them in sequential phone# order). My idea to have
PHONE+AUTONUMBER would be good because at least the phone #'s will be
sequentially next to each other on the physical disk. This database is quite
large already at 1.2 million records, and growing by 220,000 records per
month.

I really have 2 different types of records mixed together in 1 table, which
may be the root problem here (that I should redesign the schema into 2
tables!) I.E., I am storing records with phones, and also records without
phones (by the way these are sales leads purchased from InfoUSA; some have
phones which we telemarket to, and others are name/address only no phone
which we mail to). We are mixing the 2 records together in the same table
since they are both "leads". Those records with phones the obvious lookup
key is the phone (appended with an autonumber to break ties because we do
receive dupe phones occassionally in the daily feed). Those records that are
name/address only no phone, then I would rely on the autonumber to be the key
because the phone would be null or zerolengthstring

"Jamie Collins" wrote:



On Jan 5, 5:06 am, "Allen Browne" wrote:
I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique. Why
not?
Is this a bug?

Such a key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key and
'primary' has no special meaning. The UNIQUE constraint can still be
used in a FOREIGN KEY constraint (see OT note below).
Perhaps the better approach is to ask: what does PRIMARY KEY give me
that UNIQUE does not, aside from requiring that all values be not null?
(FWIW columns in the PRIMARY KEY designation can be nullable, it's just
you can't actually put the null value in them)...

The key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key.
The UNIQUE constraint can still be used in a FOREIGN KEY.

AutoNumbers are normally unique on their
own? So why would you want to use an autonumber plus another field?

A valid answer to your question is clustering or physical ordering on
disk.

The better 'phone book' example would be the desire to cluster on the
subscribers' names. The phone number (using a natural key) or
autonumber would be required to break duplicates because subscribers'
names may not be unique.

Care is required in declaring the columns in the correct order i.e.
PRIMARY KEY (name, phone_number) i.e. the columns should be in
left-to-right order of significance to the clustering.

Clustering on name would favour BETWEEN (e.g. grab all names beginning
with 'B') and GROUP BY on the name because the rows would be physically
contiguous (and on the same page) to begin with and would be faster to
fetch. Think how 'useful' a paper phone book physically order on
telephone number would be to the average user.

Clustering on phone number or *random* autonumber on the other hand
would favour concurrency but a sequential autonumber PRIMARY KEY may be
the worst choice in any case.

[OT ]

FWIW nulls in foreign key constraints is one of the few areas where the
Jet implementation is IMO better than SQL Server's (and, Allen, I know
you are a fan of the 'CASCADE to null' feature in Jet 4.0 g).

SQL Server will disregard all values in the referencing columns if one
of then contains a null and also will not cascade them e.g. from a null
value to a not null value. When you see how Jet handles it, it makes
the SQL Server approach seem very odd e.g.

CREATE TABLE Table1 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER, UNIQUE (key_col1, key_col2)
)
;
CREATE TABLE Table2 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER,
FOREIGN KEY (key_col1, key_col2)
REFERENCES Table1 (key_col1, key_col2)
ON DELETE CASCADE
ON UPDATE CASCADE
)
;

INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (3, NULL)
;

That last insert would fail in Jet but in SQL Server it would succeed
on the logic, I guess, that Table1.col2 = Table2.col2 returns unknown
therefore should not fail; Jet's apparent logic 'NULL must equal NULL'
is harder to defend 'relationally'. However, what seems strange IMO is
the SQL Server CASCADE behaviour:

UPDATE Table1
SET key_col2 = 1
WHERE key_col1 = 1
;

The above change does not cascade to the referencing table (Table2) in
SQL Server. While the inability to compare null values between tables
may be sound 'relationally' it would seem to be unintuitive and not the
desired effect. Similarly, when deleting:

DELETE
FROM Table1
WHERE key_col1 = 2

the row is removed from the referencing table in Jet but not in SQL
Server.

I think the Jet behaviour has more utility. Such 'practical' decisions
are hardly unprecedented, even my the famously strict ANSI SQL
standards committees. For example, if one null value cannot be compared
as being equal to another null value then why do they GROUP together?
e.g.

DELETE FROM Table1
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
SELECT key_col2, COUNT(*)
FROM Table1
GROUP BY key_col2
;

The answer is, at least in part, because most people would expect it to
work this way and such an approach has the greater practical utility.

Jamie.

--

nickh wrote:

So, Jamie, if I am understanding your points, you are saying that there
actually is a benefit to my idea in terms of the physical fragmentation on
the disk. I.E. the worst arrangement is to have the autonumber alone be the
primary key because it is completely random and irrelevant and there will be
the most disk head movement as I try to fetch phone#'s from the table
(assuming I am fetching them in sequential phone# order). My idea to have
PHONE+AUTONUMBER would be good because at least the phone #'s will be
sequentially next to each other on the physical disk. This database is quite
large already at 1.2 million records, and growing by 220,000 records per
month.

Yes, that's the theory. However, like anything relating to
'performance', you will need to test to determine what works best in
your particular circumstances e.g. something that benefits one query
may penalise another so you have to make a judgement on that which is
critical path. A physical in index may be 'better' than a logical one
but you will use both flavours (knowingly or otherwise). Be wary of
anyone (including me g) who suggests they may be able to determine
what is best for you based on the little information you have posted
here.

Note the mdb must be compacted for the 'clustered' index to be rebuilt.
See the following Microsoft articles:

ACC2000: Defragment and Compact Database to Improve Performance

http://support.microsoft.com/default...b;en-us;209769

"If a primary key exists in the table, compacting re-stores table
records into their Primary Key order. This provides the equivalent of
Non-maintained Clustered Indexes, and makes the read-ahead capabilities
of the Microsoft Jet database engine much more efficient."

New Features in Microsoft Jet Version 3.0

http://support.microsoft.com/default...b;en-us;137039

"Compacting the database now results in the indices being stored in a
clustered-index format. While the clustered index isn't maintained
until the next compact, performance is still improved."

I really have 2 different types of records mixed together in 1 table, which
may be the root problem here (that I should redesign the schema into 2
tables!) I.E., I am storing records with phones, and also records without
phones (by the way these are sales leads purchased from InfoUSA; some have
phones which we telemarket to, and others are name/address only no phone
which we mail to). We are mixing the 2 records together in the same table
since they are both "leads".

Consider 'subclassing', where the attributes common to all 'leads' are
found in a 'superclass' table. Do a google search on the exact phrase
"CREATE TABLE Vehicles".

Jamie.

--

I'll just mention that a many people, including Access MVPs, consider
autonumber to be a perfectly acceptable and efficient choice for PK.
However, even if physical ordering on the disk has enough of an effect on
performance that multi-field PKs are worth the extra effort, it may be worth
asking yourself if the order of phone numbers is less "random" than the
order of autonumbers. It's true that phone numbers have area codes and
exchanges so they can be grouped by state, etc., but does that help you?

"nickh" wrote in message
...
So, Jamie, if I am understanding your points, you are saying that there
actually is a benefit to my idea in terms of the physical fragmentation on
the disk. I.E. the worst arrangement is to have the autonumber alone be
the
primary key because it is completely random and irrelevant and there will
be
the most disk head movement as I try to fetch phone#'s from the table
(assuming I am fetching them in sequential phone# order). My idea to have
PHONE+AUTONUMBER would be good because at least the phone #'s will be
sequentially next to each other on the physical disk. This database is
quite
large already at 1.2 million records, and growing by 220,000 records per
month.

I really have 2 different types of records mixed together in 1 table,
which
may be the root problem here (that I should redesign the schema into 2
tables!) I.E., I am storing records with phones, and also records without
phones (by the way these are sales leads purchased from InfoUSA; some have
phones which we telemarket to, and others are name/address only no phone
which we mail to). We are mixing the 2 records together in the same table
since they are both "leads". Those records with phones the obvious lookup
key is the phone (appended with an autonumber to break ties because we do
receive dupe phones occassionally in the daily feed). Those records that
are
name/address only no phone, then I would rely on the autonumber to be the
key
because the phone would be null or zerolengthstring

"Jamie Collins" wrote:



On Jan 5, 5:06 am, "Allen Browne" wrote:
I have 2 fields: [PHONE] and [AUTONUMBER]

I would like to define the Primary Key as a multi-field primary key
as the
concatenation of [PHONE]+[AUTONUMBER]

Now, [PHONE] may be null, but [AUTONUMBER] will always contain a
unique
autonumber.

The problem is that Access won't allow null in [PHONE] even though
the
multi-field concatenation of [PHONE]+[AUTONUMBER] would be unique.
Why
not?
Is this a bug?

Such a key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key and
'primary' has no special meaning. The UNIQUE constraint can still be
used in a FOREIGN KEY constraint (see OT note below).
Perhaps the better approach is to ask: what does PRIMARY KEY give me
that UNIQUE does not, aside from requiring that all values be not null?
(FWIW columns in the PRIMARY KEY designation can be nullable, it's just
you can't actually put the null value in them)...

The key could implemented using a UNIQUE constraint i.e.
UNIQUE([PHONE], [AUTONUMBER]). In relational terms, a key is a key.
The UNIQUE constraint can still be used in a FOREIGN KEY.

AutoNumbers are normally unique on their
own? So why would you want to use an autonumber plus another field?

A valid answer to your question is clustering or physical ordering on
disk.

The better 'phone book' example would be the desire to cluster on the
subscribers' names. The phone number (using a natural key) or
autonumber would be required to break duplicates because subscribers'
names may not be unique.

Care is required in declaring the columns in the correct order i.e.
PRIMARY KEY (name, phone_number) i.e. the columns should be in
left-to-right order of significance to the clustering.

Clustering on name would favour BETWEEN (e.g. grab all names beginning
with 'B') and GROUP BY on the name because the rows would be physically
contiguous (and on the same page) to begin with and would be faster to
fetch. Think how 'useful' a paper phone book physically order on
telephone number would be to the average user.

Clustering on phone number or *random* autonumber on the other hand
would favour concurrency but a sequential autonumber PRIMARY KEY may be
the worst choice in any case.

[OT ]

FWIW nulls in foreign key constraints is one of the few areas where the
Jet implementation is IMO better than SQL Server's (and, Allen, I know
you are a fan of the 'CASCADE to null' feature in Jet 4.0 g).

SQL Server will disregard all values in the referencing columns if one
of then contains a null and also will not cascade them e.g. from a null
value to a not null value. When you see how Jet handles it, it makes
the SQL Server approach seem very odd e.g.

CREATE TABLE Table1 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER, UNIQUE (key_col1, key_col2)
)
;
CREATE TABLE Table2 (
key_col1 INTEGER NOT NULL,
key_col2 INTEGER,
FOREIGN KEY (key_col1, key_col2)
REFERENCES Table1 (key_col1, key_col2)
ON DELETE CASCADE
ON UPDATE CASCADE
)
;

INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table2 (key_col1, key_col2) VALUES (3, NULL)
;

That last insert would fail in Jet but in SQL Server it would succeed
on the logic, I guess, that Table1.col2 = Table2.col2 returns unknown
therefore should not fail; Jet's apparent logic 'NULL must equal NULL'
is harder to defend 'relationally'. However, what seems strange IMO is
the SQL Server CASCADE behaviour:

UPDATE Table1
SET key_col2 = 1
WHERE key_col1 = 1
;

The above change does not cascade to the referencing table (Table2) in
SQL Server. While the inability to compare null values between tables
may be sound 'relationally' it would seem to be unintuitive and not the
desired effect. Similarly, when deleting:

DELETE
FROM Table1
WHERE key_col1 = 2

the row is removed from the referencing table in Jet but not in SQL
Server.

I think the Jet behaviour has more utility. Such 'practical' decisions
are hardly unprecedented, even my the famously strict ANSI SQL
standards committees. For example, if one null value cannot be compared
as being equal to another null value then why do they GROUP together?
e.g.

DELETE FROM Table1
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (1, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (2, NULL)
;
INSERT INTO Table1 (key_col1, key_col2) VALUES (3, 3)
;
SELECT key_col2, COUNT(*)
FROM Table1
GROUP BY key_col2
;

The answer is, at least in part, because most people would expect it to
work this way and such an approach has the greater practical utility.

Jamie.

--

BruceM wrote:

I'll just mention that a many people, including Access MVPs, consider
autonumber to be a perfectly acceptable and efficient choice for PK.

I'll just mention that many people, including the Emperor's most
trusted advisors, considered his New Clothes to be a perfectly
acceptable choice for a state occasion g.

Researching a natural key can involve time and effort (though usually a
google search will quickly reveal an industry-standard key, ISO
standard, etc). Designing your own key can be hard work. Is there
anything simpler than incrementing an integer? Sure, an integer that is
incremented for you!

It is mere coincidence in this case that 'the easy way out' is the not
'the road less travelled'?

Jamie.

--