Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 8: Being Normal: Normalization and Other Basic Design Issues
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UnitPrice is relatively easy: As this field is going to hold a monetary value, its data type hints at
money. Be careful with this though. How many decimal places do you really need? U.S. developers are
going to tend toward thinking they only need two (several currencies often trade in more decimal places),
but what about commodity items? For example, gasoline is generally sold in fractions of a cent. The
money data type handles 4 decimal places, which meets most currency needs.
Moving along, we’re again (no surprise here) considering all data fields to be required. No, we’re not
allowing nulls anywhere.
No defaults seem to make sense for this table, so we’re skipping that part also.
Identity? The temptation might be to mark OrderID as an identity column again. Don’t do that!
Remember that OrderID is a value that we’re going to match to a column in another table. That table
will already have a value (as it happens, set by identity, but it didn’t necessarily have to be that way),
so setting our column to identity would cause a collision. We would be told that we can’t do our insert
because we’re trying to set an identity value. All the other columns either get their data from another
table or require user input of the data. IsRowGuid does not apply again.
That takes us to our Products and Customers tables, as shown in Figures 8-26 and 8-27 respectively.
Figure 8-26
Figure 8-27
Let’s hit the highlights on the choices here and move on.
PartNo has been defined by the data that we saw when we were looking at normalization. It’s a numeric,
followed by an alpha, followed by four numerics. That’s six characters, and it seems to be fixed. We would
want to hold the customer to the cross about the notion that the size of the part number can’t get any
larger but, assuming that’s OK, we’ll go with a char(6) here. That’s because a char takes up slightly
less overhead than a varchar, and we know that the length is going to always remain the same (this
means that there’s no benefit from the variable size).