SQL Server order by syntax with Case When and a Constant - sql-server

Question

I'm reading TSQL code someone else wrote and find a somewhat weird syntax. It's doing order by a string. I did some test and the following is the code. Anyone can help me to explain it? Thanks.
First Query
SELECT *
FROM dbo.Products
Result:
ProductID ProductName SupplierID CategoryID QuantityPerUnit UnitPrice UnitsInStock UnitsOnOrder ReorderLevel Discontinued
----------- ------------------------------- ----------- ----------- -------------------- --------------------- ------------ ------------ ------------ ------------
1 Chai 1 1 10 boxes x 20 bags 18.00 39 0 10 0
2 Chang 1 1 24 - 12 oz bottles 19.00 17 40 25 0
3 Aniseed Syrup 1 2 12 - 550 ml bottles 10.00 13 70 25 0
4 Chef Anton's Cajun Seasoning 2 2 48 - 6 oz jars 22.00 53 0 0 0
...
*/
Second query:
SELECT *
FROM dbo.Products
WHERE ProductID < 10
ORDER BY '3';
Result:
Msg 408, Level 16, State 1, Line 1 A constant expression was
encountered in the ORDER BY list, position 1.
Third Query
SELECT *
FROM dbo.Products
WHERE ProductID < 10
ORDER BY CASE WHEN SupplierID = 2 THEN '1'
WHEN SupplierID = 1 THEN '2'
ELSE '3'
END;
Result:
ProductID ProductName SupplierID CategoryID QuantityPerUnit UnitPrice UnitsInStock UnitsOnOrder ReorderLevel Discontinued
----------- ---------------------------------------- ----------- ----------- -------------------- --------------------- ------------ ------------ ------------ ------------
4 Chef Anton's Cajun Seasoning 2 2 48 - 6 oz jars 22.00 53 0 0 0
5 Chef Anton's Gumbo Mix 2 2 36 boxes 21.35 0 0 0 1
1 Chai 1 1 10 boxes x 20 bags 18.00 39 0 10 0
2 Chang 1 1 24 - 12 oz bottles 19.00 17 40 25 0
3 Aniseed Syrup 1 2 12 - 550 ml bottles 10.00 13 70 25 0
6 Grandma's Boysenberry Spread 3 2 12 - 8 oz jars 25.00 120 0 25 0
7 Uncle Bob's Organic Dried Pears 3 7 12 - 1 lb pkgs. 30.00 15 0 10 0
8 Northwoods Cranberry Sauce 3 2 12 - 12 oz jars 40.00 6 0 0 0
9 Mishi Kobe Niku 4 6 18 - 500 g pkgs. 97.00 29 0 0 1
(9 row(s) affected)
*/

Answer 1

"Order by" has to be able to translate each row into a value, then those values can be compared. "Order by '3'" doesn't make any sense as a useful query, as it's not using the row - hence the error message of ordering by a constant expression.
"Order by (some expression returning a string)" makes perfect sense. I would personally have used numbers rather than strings, but fundamentally it's still just ordering by a value.
Would you have found it odd to see "order by ProductName"? That's ordering by a string too.
Hopefully that helps - it's not really clear which bit was causing a problem though.

Answer 2

In your first SQL query, the original coder might have meant
ORDER BY 3
which means "order by the 3rd column" (which is SupplierId), in ascending order.
In the second query, as #Kokizzo has explained, the author has hard coded the query so that Products from supplierId 2 are at the top, followed by those from supplierId 1, and then all rows from other suppliers. The purpose isn't clear, but for example, this could be a nefarious attempt to promote a certain supplier's products above others e.g. in a web search result page.
The CASE WHEN .. ELSE ... END can be equated to a simple function applied to each row, which takes the supplierId as input and returns the precedence of that row used in the ORDER BY clause.

Answer 3

CASE WHEN SupplierID = 2 THEN '1'
WHEN SupplierID = 1 THEN '2'
ELSE '3'
END
is equal to (in pseudocode):
if supplierId = 2 then
order_value = 1
else if supplierId = 1 then
order_value = 2
else
order_value = 3
end
so the order now is according to the order_value