Transact-SQL Select statement results with bad GUID - sql

Question

We have a table with GUID primary keys. When I search for a specific key, I can use either:
SELECT * FROM products WHERE productID='34594289-16B9-4EEF-9A1E-B35066531DE6'
SELECT * FROM products WHERE productID LIKE '34594289-16B9-4EEF-9A1E-B35066531DE6'
RESULT (for both):
product_ID Prd_Model
------------------------------------ --------------------------------------------------
34594289-16B9-4EEF-9A1E-B35066531DE6 LW-100
(1 row affected)
We have a customer who uses our ID but adds more text to it to create some kind of compound field in their own system. They sent me one of these values to look up and I had an unexpected result. I meant to trim the suffix but forgot, so I ran this:
SELECT * FROM products WHERE productID='34594289-16B9-4EEF-9A1E-B35066531DE6_GBR_USD'
When I ran it, I unexpectedly got the same result:
product_ID Prd_Model
------------------------------------ --------------------------------------------------
34594289-16B9-4EEF-9A1E-B35066531DE6 LW-062
(1 row affected)
Now if I trim a value off the end of the GUID when searching I get nothing (GUID is 1 digit short):
SELECT * FROM products WHERE productID='34594289-16B9-4EEF-9A1E-B35066531DE'
Result:
product_ID Prd_Model
------------------------------------ --------------------------------------------------
(0 rows affected)
When using the LIKE command instead of '=' and if I add the suffix to the end, the statement returns zero results. This is what I would expect.
So why does the longer string with the suffix added to the end return a result when using '=' in the statement? It's obviously ignoring anything beyond the 36-character GUID length but I'm not sure why.

Answer 1

This behaviour is documented:
Converting uniqueidentifier Data
The uniqueidentifier type is considered a character type for the purposes of conversion from a character expression, and therefore is subject to the truncation rules for converting to a character type. That is, when character expressions are converted to a character data type of a different size, values that are too long for the new data type are truncated. See the Examples section.
So, the string value '34594289-16B9-4EEF-9A1E-B35066531DE6_GBR_USD' is truncated to '34594289-16B9-4EEF-9A1E-B35066531DE6' when it is implicitly cast (due to Data Type Precedence) to a uniqueidentifier and, unsurprisingly, '34594289-16B9-4EEF-9A1E-B35066531DE6' equals itself so the row is returned.
And the documentation does indeed give an example:
The following example demonstrates the truncation of data when the value is too long for the data type being converted to. Because the uniqueidentifier type is limited to 36 characters, the characters that exceed that length are truncated.
DECLARE #ID NVARCHAR(max) = N'0E984725-C51C-4BF4-9960-E1C80E27ABA0wrong';
SELECT #ID, CONVERT(uniqueidentifier, #ID) AS TruncatedValue;
Here is the result set.
String TruncatedValue
-------------------------------------------- ------------------------------------
0E984725-C51C-4BF4-9960-E1C80E27ABA0wrong 0E984725-C51C-4BF4-9960-E1C80E27ABA0
I, however, find it odd that you say that the statement below returns no rows:
SELECT *
FROM products
WHERE productID='34594289-16B9-4EEF-9A1E-B35066531DE'
Though true, it won't return rows, it will also error:
Conversion failed when converting from a character string to uniqueidentifier.
The fact it doesn't implies your column isn't a uniqueidentifier which would mean that your first statement isn't true; as the longer string would not be truncated. This means that one of the statements in the question is likely wrong; either your column is a uniqueidentifier and thus you get results but get an error in the latter, or it isn't and neither statement would return a result set. As you can see in this demonstration:
CREATE TABLE dbo.YourTable (UID uniqueidentifier, String varchar(36));
INSERT INTO dbo.YourTable (UID,String)
VALUES('34594289-16B9-4EEF-9A1E-B35066531DE6','34594289-16B9-4EEF-9A1E-B35066531DE6');
GO
--Returns data
SELECT *
FROM dbo.YourTable
WHERE UID = '34594289-16B9-4EEF-9A1E-B35066531DE6_GBR_USD'
GO
--Errors
SELECT *
FROM dbo.YourTable
WHERE UID = '34594289-16B9-4EEF-9A1E-B35066531DE';
GO
--Returns no data
SELECT *
FROM dbo.YourTable
WHERE String = '34594289-16B9-4EEF-9A1E-B35066531DE6_GBR_USD'
GO
--Returns no data
SELECT *
FROM dbo.YourTable
WHERE String = '34594289-16B9-4EEF-9A1E-B35066531DE'
GO
DROP TABLE dbo.YourTable;
db<>fiddle