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

Chapter 8: Being Normal: Normalization and Other Basic Design Issues
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OK, we’ve got our CustomerNotes table into the database, but now we notice a problem — the way our
primary key is declared, we can only have only one note per customer. More than likely, we are going to
keep taking more and more notes on the customer over time. That means that we need to change our
primary key, and leaves us with a couple of options depending on our requirements:
❑ Make the date part of our primary key: This is problematic from two standpoints. First, we’re
tracking which employee took the note — what if two different employees wanted to add notes
at the same time? We could, of course, potentially address this by also adding EmployeeID to
the primary key. Second, what’s to say that even the same employee wouldn’t want to enter two
completely separate notes on the same day (OK, so, since this is a datetime field, they could do
it as long as they didn’t get two rows inserted at the same millisecond — but just play along
with me here)? Oops, now even having the EmployeeID in the key doesn’t help us.
❑ Add another column to help with the key structure: We could either do this by adding a counter
column for each note per customer. As yet another alternative, we could just add an identity column
to ensure uniqueness — it means that our primary key doesn’t really relate to anything, but
that isn’t always a big deal (though it does mean that we have one more index that has to be
maintained) and it does allow us to have a relatively unlimited number of notes per customer.
I’m going to take the approach of adding a column I’ll call “Sequence” to the table. By convention (it’s
not a requirement and not everyone does it this way), primary keys are normally the first columns in
your table. If we were going to be doing this by script ourselves, we’d probably just issue an ALTER TABLE
statement and ADD the column — this would stick our new column down at the end of our column list.
If we wanted to fix that, we’d have to copy all the data out to a holding table, drop any relationships to
or from the old table, drop the old table, CREATE a new table that has the columns and column order we
want, then re-establish the relationships and copy the data back in (a long and tedious process). With the
diagramming tools, however, SQL Server takes care of all that for us.
To insert a new row in the middle of everything, I just right-click on the row that is to immediately
follow the row I want to insert and select Insert Column. The tool is nice enough to bump everything
down for me to create space just like Figure 8-12.
Figure 8-12