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I've spent much of my adult life designing and implementing
relational databases for fun and profit. One of the most important
aspects of designing a relational database is to fully understand the
relationships between the various tables. The best tool for this is
to develop a schema diagram that graphically illustrates these
relationships using "crow's feet notation".
Crow's feet notation defines graphical three elements (O, dash and
crow foot) that may be combined to define four possible constraints
to a side of the relationship:
 One and Only One
 Zero or One
 Zero through Many
 One through Many
Let's say we have a very simple database to track contact
information with the following conditions:
- We rely on email, so each person MUST have at least one email
address associated with him
- Telephone is not so important, so each person may have several
telephone numbers (office, home, cell, fax, etc)
- Each person may more may not be associated with a company
The following diagram illustrates a database schema to meet these
requirements:
The phonetype and emailtype tables are static reference tables that
would be preloaded with vlues like "Home, Office, Mobile, Fax", etc.
For each contact in the "people" table there are "zero to many" telephone
numbers and, conversly, for every telephone number there must be at
least one person associated with it (for e.g. a business number or shared
home number). Email addresses can not be shared, so for each email
address there is one and only one person.
You get the idea...
I used to created these diagrams by hand, using trusty old pencil and
paper. Recently I decided to give OpenOffice.org's Draw a try at this.
One of the problems with Draw is the lack of these symbols as arrows
for the connectors. Fortunately creating arrows in Draw is very easy.
Simply define the graphic as a polygon and assign it to an arrow!
Crows_Feet_Arrows.odg is
an OpenOffice.org file that contains polygon graphics of each of the
crow's feet elements. If you're building a database schema diagram
simply load this file into Draw and for each polygon select it then
right-mouse-click and select the Line option from the context
menu. In the Line dialogue select the Arrow Styles tab, click the
Add button then enter a name for the arrow (e.g. Crow's Feet -
Zero through Many) and click the OK button. Your new arrow will now be
listed in the Arrow Style drop down list. Once you have them all set up
you can save the arrow styles to the default standard.soe file and they
will be there for you to continue using.
Enjoy!
Jay MacDonald
June 2007
For more information on Crow's Feet notation and how they are used
visit this
page on Wikipedia. For more information on creating arrows in
OpenOffice.org visit
Solveig Haugland's blog page about it.
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