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

Chapter 11: Writing Scripts and Batches
Setting Variables Using SET
SET is usually used for setting variables in the fashion that you would see in more procedural languages.
Examples of typical uses would be:
SET @TotalCost = 10
SET @TotalCost = @UnitCost * 1.1
Notice that these are all straight assignments that use either explicit values or another variable. With a
SET, you cannot assign a value to a variable from a query — you have to separate the query from the
SET. For example:
USE AdventureWorks2008;
DECLARE @Test money;
SET @Test = MAX(UnitPrice) FROM [Order Details];
SELECT @Test;
causes an error, but:
USE AdventureWorks2008;
DECLARE @Test money;
SET @Test = (SELECT MAX(UnitPrice) FROM Sales.SalesOrderDetail);
SELECT @Test;
works just fine.
Although this latter syntax works, by convention, code is never implemented this way. Again, I don’t
know for sure why it’s “just not done that way,” but I suspect that it has to do with readability — you
want a SELECT statement to be related to retrieving table data, and a SET to be about simple variable
assignments.
Setting Variables Using SELECT
SELECT is usually used to assign variable values when the source of the information you’re storing in
the variable is from a query. For example, our last illustration would be typically done using a SELECT:
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USE AdventureWorks2008;
DECLARE @Test money;
SELECT @Test = MAX(UnitPrice) FROM Sales.SalesOrderDetail;
SELECT @Test;
Notice that this is a little cleaner (it takes less verbiage to do the same thing).
So again, the convention on when to use which goes like this:
❑ Use SET when you are performing a simple assignment of a variable — where your value is
already known in the form of an explicit value or some other variable.
❑ Use SELECT when you are basing the assignment of your variable on a query.