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

Chapter 7: Adding More to Our Queries
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So, the question probably quickly rises in your mind, “Hey, if CONVERT does everything that CAST
does, and CONVERT also does date conversions, why would I ever use CAST?” I have a simple answer
for that — ANSI/ISO compliance. CAST is ANSI/ISO-compliant, but CONVERT isn’t — it’s that simple.
Let’s take a look for the syntax for each.
CAST (expression AS data_type)
CONVERT(data_type, expression[, style])
With a little flip-flop on which goes first and the addition of the formatting option on CONVERT (with the
style argument), they have basically the same syntax.
CAST and CONVERT can deal with a wide variety of data-type conversions that you’ll need to do when
SQL Server won’t do it implicitly for you. For example, converting a number to a string is a very common
need. To illustrate:
SELECT ‘The Customer has an Order numbered ‘ + SalesOrderID
FROM Sales.SalesOrderHeader
WHERE CustomerID = 29825;
will yield an error:
Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value ‘The Customer has an Order
numbered ‘ to data type int.
But change the code to convert the number first:
SELECT ‘The Customer has an Order numbered ‘ + CAST(SalesOrderID AS varchar)
FROM Sales.SalesOrderHeader
WHERE CustomerID = 29825;
And you get a much different result:
-----------------------------------------------------------------
The Customer has an Order numbered 43659
The Customer has an Order numbered 44305
The Customer has an Order numbered 45061
The Customer has an Order numbered 45779
The Customer has an Order numbered 46604
The Customer has an Order numbered 47693
The Customer has an Order numbered 48730
The Customer has an Order numbered 49822
The Customer has an Order numbered 51081
The Customer has an Order numbered 55234
The Customer has an Order numbered 61173
The Customer has an Order numbered 67260
(12 row(s) affected)