I find that using paper, and drawing out my designs really helps.
I don't setup relationships, or any thing, but simply start with a blank
piece of paper.
If I need something called customers,then I draw a big box, and call it
customers. I will then note that customers need invoices, so then I will
draw a big box, and name it Invoices. I will then draw a line with a arrow
head from customer box to invoices box (customers -----> invoices.)
The draw often looks very child like. Note that I don't mention field names,
or any kind of mumbo-jumbo. Just a easy to draw diagram. It turns out that
visual minds tend to be great at data modeling, but you just got to make it
a visual process!!
If fact, if you are a up to speed with Excel, then try using the 'drawing'
tools in excel (the ones for flow charts are really nice. In fact, they are
better then "word" drawing, since when you move around the boxes, the "join"
lines will also move. Drawing cute squares, giving each square a name, and
connecting them is very easy and "fun". No funny relationships stuff, just
squares and lines!!
The above should help you. The only thing to remember is that a table can
have a relationship to another table. It is either one to one, or one to
many. That is the ONLY choice you got....
I will also "re-post" a old post of mine where I explain how I use the
"visual" cues of a the relationships diagram to "guide" my lost brain.....
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For forms, and sub-forms, and related tables, left joins are quite
important.
If you look at the following screen shot, you can see that most relations
ships are this left join, and RI is enforced.
http://www.members.shaw.ca/AlbertKallal/Articles/PickSql/Appendex2.html
tblBgroup (booking group) for example may, or may not have payments made
(tblPayments). Thus, you can add a booking group, and NOT have to add child
records. However, full RI is enforced, and you can see the side ways 8
"omega" sign AND THE ARROW HEAD. The simple lookup fields are simply just a
arrow drawn, and no "1", or omega sign exists (tblPayments to tblHowpaid for
example is a simple lookup). It is GREAT that I can look at the ER diagram,
and instantly know if child records are required, or they are not!!
The tables that MUST have a child records can also clearly be seen. If you
go from the tblBgroup to the its parent table, you will see table
tblBooking. You can easily see that there is a 1 to many here also, but NO
ARROW head exists. Thus, when I create a booking, my designs will ALWAYS
ASSUME that a child records in tblBgroup (booking group) will exist (ie: I
must code, and assume that when I add a tblBooking records, my code also
assumes that a tblBGroup will also have to be added). In plain English this
means that when I make a booking (reservation), my code assumes that
you MUST have people in that booking. However, I most certainly allow
people to be booked, but not yet have made any payments. So, your
relationship(s) if done right should reflect the rules you as a developer
want to maintain. I should point out that a left join, or a standard
(inner join) both allow child records to NOT exist, but you still
should correctly set this relationship, since when it comes to making
reports, and writing code...I will know what my assumptions
were at the time (ie: do I HAVE to add those child records
for the software to function correctly. So, if I write code to
make a booking, all of my code thus assumes that people
are also to be added to the booking. Break that assuming
of mine, and likely my code will break).
So, the ER diagram can convey a lot about your designs. Down the road, I can
now look at that diagram, and when writing code, I will know if the design
can, and does assume if child records are required. If you look at that
table, it is VERY RARE that I require the child record. That application has
about 60 tables, and I think only 1 or 2 in the whole thing is NOT a left
join. Hence, you most certainly should set the relation in the window for
future reference, and also it will help you when you create a query, or a
report.
The ER window in fact turns the development process into a "visual" one, and
that is how my brain works...