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

Chapter 15: Triggers
The COLUMNS_UPDATED() Function
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This one works somewhat differently from UPDATE(), but has the same general purpose. What COLUMNS_
UPDATED() gives us is the ability to check multiple columns at one time. In order to do this, the function
uses a bitmask that relates individual bits in one or more bytes of varbinary data to individual columns
in the table. It ends up looking something like Figure 15-4.
Figure 15-4
In this case, our single byte of data is telling us that the second, third, and sixth columns were
updated — the rest were not.
In the event that there are more than eight columns, SQL Server just adds another byte on the right-hand
side and keeps counting (see Figure 15-5).
0 1
Figure 15-5
0 1
Column 1
Column 2
Column 3
Column 4
Column 5
Column 6
Column 7
Column 8
0 0 0 0 0
First Byte Second Byte
This time the second, ninth, and fourteenth columns were updated.
0
Column 1, value = 1
Column 2, value = 2
Column 3, value = 4
Column 4, value = 8
Column 5, value = 16
Column 6, value = 32
Column 7, value = 64
Column 8, value = 128
1 0 0 1 0 0
1 0
Column 9
Column 10
Column 11
Column 12
Column 13
Column 14
Column 15
Column 16
0 0 0 1 0 0
I can hear you out there: “Gee, that’s nice — but how do I make any use of this?” Well, to answer that,
we have to get into the world of boolean algebra.
Making use of this information means that you need to add up the binary value of all the bytes, considering
the leftmost digit to be the least significant. So, if you want your comparison to take into account 2,