| Conceptual relationships are a starting point for
RE defining physical relationships.

A RA ALA

TRE raje s — nn rji

parts. We are missing the record for a pur-
chase order's line item. Therefore, the dia-
gram would look something like this:

Z PURCHORO era :

Each purchase order may include
many line items, but each line item is on
Only one PO. Similarly, a part number can
be on many line items (in different POs), but
each line item is for only one part number.

In short, a two-headed arrow is

meaningful because it indicates a many-to-.

many relationship. but it is a symptom that
there's an entity missing. A hidden record
exits which, when found, forms the inter-
section of the two records already identi-

One last thought: whenever a two-
headed arrow is replaced with a new inter-
section entity, review the other relation:
ships in which the original parent entities
were involved. Look at each arrow pointing
toward the parent boxes and consider its
meaning. You may find it is more appropri-
ately drawn pointing to the newly formed
intersection box than to the original parent
box. The relationship still carries the same
meaning, but moving it to point to the new
box makes the diagram more precise.

Each inspection department can be
involved with many purchase orders. The
fact that their involvement ("inspection
date," "guantity rejected," etc.) is at the
level of individual line items can be re-
vealed in three steps.

Start by identifying the many-to-

INSPECTION
DEPARTMENT

PURCHORD bi ih PARTNUM

Reconsider the relationship between
inspection department and purehase order.

Ri

INSPECTION
DEPARTMENT

PUACHORO Hay €] PARTNUM

fied. When you find a many-to-many situ-
ation in file design, feel free to tag it with
ATTOWS at each end. Remember, though, it
Hi means we are not finished. Find the missing
K intersection record, break the arrow in two,
h insert the intersection, reverse the broken
m: pieces of arrow, and the two-headed rela-
tionship will vanish.

HI

h

RTV. toni gt tree ikone

Next time, we'll review the second
basic manipulation, merging entities with
the headless arrow. e

INSPECTION
DEPARTMENT

many relationship.
ad
Mi PARTNUM

Identify and draw the intersection
entity.

a

Frank Sweet is corporate manager of
data acministration for the Charter Co.,
Jacksorville, Fla.

šč pali k? K : l ing? ROM,
ori

Ba st and geographe area?
tumover rate for your

« the average dp
s the trend in benefit
e the pay scale rang?

e the annual employ'
or location?

DATAMATION research reports have
the answers. We regularly survey our
183.000 readers around the world about
their operations, and detailed statistical
reports of these studies are now available.

The 1984 DATAMATION Salary Survey,
including 160 pages of tables covering
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areas and 11 different industry catego-
ries. is available for $100. The 1984
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and executive summary, including 261
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industry categories and 16 line items of
the typical DP budget, is only $100.

For further information contact

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Technical Publishing Co., 875 Third Ave.,
New York. NY 10022.

TO: Laurie Sehnepf, Research Director, Technical Publishing
875 Third Ave., New York, NY 10022.

Please send me copies of:
—— he 1984 DATAMATION Salary Survey, at $100 each.
——he 1984 DATAMATION Budget Survey, at $100 each.

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! 138 -GAMALON