What code is needed to conditionally provide a value to a field in a beforeinsert trigger? - sql-server

Question

I need to store a string value in a field in a table, specifically in its Subcategory VarChar(50) column.
The value of Subcategory prior to this post processing is either 0 or 1; I need to change that to a more human-friendly value.
I haven't created a database trigger in decades and need some help with the code. This is my pseudo SQL (a hodgepodge of SQL and VB):
CREATE OR REPLACE TRIGGER tr_CustomerCategoryLog_BeforeInsert
BEFORE INSERT ON CustomerCategoryLog FOR EACH ROW
DECLARE _Category = :new.Category;
DECLARE _Subcategory = :new.Subcategory;
BEGIN
If _Category = "New"
If _Subcategory = 0
:new.Subcategory := 'New';
End If
Else If _Subcategory = 1
:new.Subcategory := 'Assumed';
End If
End If
If _Category = "Existing"
If _Subcategory = 0
:new.Subcategory := 'Existing';
End If
Else If _Subcategory = 1
:new.Subcategory := 'Organic'
End If
End If
Return "Unknown"
End Function
END;
If the logic isn't clear, in semi-plain English it is:
If the value of the Category field is "New", set the Subcategory field value also to "New" if the value of Subcategory is currently 0; otherwise, set it to "Assumed"
If the value of the Category field is "Existing", set the Subcategory field value also to "Existing" if the value of Subcategory is currently 0; otherwise, set it to "Organic"
Maybe I need to give Steely Dan's album "Trigger Logic" a listen.
UPDATE
I think the answer will work, but it's not complete enough for me.
Since I apparently have Oracle code mixed up in the pseudoSQL above, what would the complete code need to look like (to create a BeforeInsert trigger on the CustomerCategoryLog table)?
Is this more like it:
CREATE TRIGGER tr_CustomerCategoryLog_BeforeInsert
ON CustomerCategoryLog
INSTEAD OF INSERT
AS
BEGIN
SELECT
CASE
WHEN #Category = 'New'
THEN CHOOSE(#Subcategory + 1, 'New', 'Assumed')
WHEN #Category = 'Existing'
THEN CHOOSE(#Subcategory + 1, 'Existing', 'Organic')
ELSE 'Unknown'
END
END
?

Answer 1

I tend to avoid triggers (perhaps a character flaw... I also don't like mashed potatoes), but the following illustration could simplify your logic
Declare #Category varchar(50) = 'Existing'
Declare #Subcategory int = 1 -- works if BIT
Select case when #Category = 'New' then choose(#Subcategory+1,'New','Assumed')
when #Category = 'Existing' then choose(#Subcategory+1,'Existing','Organic')
else 'Unknown' end
Returns
Organic