Multi-Field Primary Key

[nickh]

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?

 

[Allen Browne]

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.

 

[nickh]

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.

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

Reply to group, rather than allenbrowne at mvps dot org.

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?

 

[John Vinson]

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]

 

[Van T. Dinh]

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]?

 

[nickh]

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

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]

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?

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

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]

So I'm the chump who's being duped by the slick-talking autonumber heretics,
is that it?
I can find all sorts of arguments pro and con on any topic, so the existence
of advocacy for one position or the other does nothing by itself to sway my
views. Autonumber works fine for my purposes, but my point, which you
ignored, was that even if I was to accept your position on "natural" keys,
what is "natural" about a phone number? Unless you are sorting by phone
number to find gaps or something like that the ordering of a phone number
field is surely as arbitrary as an autonumber field.

 

[BruceM]

Rick, thanks for the explanation. I have done some reading on the subject,
but usually get bogged down when the discussion gets into clustered indexes,
physical ordering on the disk, and the like.
I was responding in part because the OP wrote (in response to Jamie): "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)."
I wondered if adding another essentially arbitrary number (phone #) to the
key would somehow improve this situation, assuming that the part about
physical location on the disk is essentially accurate in the first place. I
would think that it would make more sense, in a case involving a surrogate
key as part of a multi-field PK, to have the natural part of the key be
something such as LastName that would actually be used for sorting and
filtering. If one is going to combine something with autonumber, from the
uniqueness point of view it doesn't much matter what that is.
Anyhow, I appreciate your adding some comments I can actually begin to
understand. By the way, who decided what is "proper"? I'm not disputing
the point so much as wondering how it came to be.

 

[Rick Brandt]

Rick, thanks for the explanation. I have done some reading on the
subject, but usually get bogged down when the discussion gets into
clustered indexes, physical ordering on the disk, and the like.
I was responding in part because the OP wrote (in response to Jamie):
"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)."
I wondered if adding another essentially arbitrary number (phone #)
to the key would somehow improve this situation, assuming that the
part about physical location on the disk is essentially accurate in
the first place. I would think that it would make more sense, in a
case involving a surrogate key as part of a multi-field PK, to have
the natural part of the key be something such as LastName that would
actually be used for sorting and filtering. If one is going to
combine something with autonumber, from the uniqueness point of view
it doesn't much matter what that is. Anyhow, I appreciate your adding some
comments I can actually begin to
understand. By the way, who decided what is "proper"? I'm not
disputing the point so much as wondering how it came to be.

The theory purists will mostly quote <genuflect> Codd and Date </genuflect>.
They pretty much "wrote the books" on relational set theory. With that
foundation to build on many "real world practioners" found that adhering too
strictly to theory can be really difficult and less productive, so for the
sake of problem solving and productivity they deviated from pure theory.

Some of these "devious" ;-) practices have become so common-place that many
now conflate them to theory in their own right. While they are common and
useful one should be aware that they are deviations from relational theory,
not part of it.

 

[David W. Fenton]

The theory purists will mostly quote <genuflect> Codd and Date
</genuflect>. They pretty much "wrote the books" on relational set
theory. With that foundation to build on many "real world
practioners" found that adhering too strictly to theory can be
really difficult and less productive, so for the sake of problem
solving and productivity they deviated from pure theory.

If you're choosing your primary keys based on ordering of the
storage, you're way outside the theoretical realm. That's an
implementation issue and is going to be answered differently for
each different database engine (e.g., Jet will be different from
MySQL).

I can't imagine a telephone number table that would *need* a PK that
didn't involve the parent foreign key.

In any event, you'd have to have hundreds of thousands of records in
the telephone table for it to make a noticeable difference.

 

[BruceM]

The OP, explaining his understanding of your comments, wrote "the worst
arrangement is to have the autonumber alone be the primary key because it is
completely random and irrelevant". I questioned whether a phone number is
less random in terms of ordering. I understand that it is natural in the
sense that it corresponds to a real-world application, but it is unlikely to
be used for ordering except maybe in the case of a call list. That being
the case, combining autonumber with phone number offers dubious advantage at
best over autonumber alone.
I understand a case may be made for natural keys, but I tend to regard
invariable rules with some suspicion.