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

Chapter 8: Being Normal: Normalization and Other Basic Design Issues
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We have a number of issues to deal with in this table if we’re going to normalize it. While we have a
functional primary key (yes, these existed long before relational systems), we have problems with both
of the main areas of the first normal form:
❑ I have repeating groups of data (customer information). I need to break that out into a different
table.
❑ The ItemsOrdered column does not contain data that is atomic in nature.
We can start by moving several columns out of the table:
OrderNo
(PK)
OrderDate CustomerNo ItemsOrdered
100 1/1/1999 54545 1A4536, Flange, 7lbs, $75;4-OR2400, Injector, .5lbs,
$108;4-OR2403, Injector, .5lbs, $116;1-4I5436, Head,
63lbs, $750
101 1/1/1999 12000 1-3X9567, Pump, 5lbs, $62.50
102 1/1/1999 66651 7-8G9200; Fan, 3lbs, $84;1-8G5437, Fan, 3lbs, $15;1-
3H6250, Control, 5lbs, $32
103 1/2/1999 54545 40-8G9200, Fan, 3lbs, $480;1-2P5523, Housing, 1lbs,
$165;1-3X9567, Pump, 5lbs, $42
And putting them into their own table:
CustomerNo (PK) CustomerName CustomerAddress
54545 ACME Co 1234 1st St.
12000 Sneed Corp. 555 Main Ave.
66651 ZZZ & Co. 4242 SW 2nd
There are several things to notice about the old and new tables:
❑ We must have a primary key for our new table to ensure that each row is unique. For our
Customers table, there are two candidate keys — CustomerNo and CustomerName. CustomerNo
was actually created just to serve this purpose and seems the logical choice — after all, it’s entirely
conceivable that you could have more than one customer with the same name. (For example,
there have to be hundreds of companies named ACME in the U.S.)
❑ Although we’ve moved the data out of the Orders table, we still need to maintain a reference to
the data in the new Customers table. This is why you still see the CustomerNo (the primary
key) column in the Orders table. Later on, when we build our references, we’ll create a foreign
key constraint to force all orders to have valid customer numbers.