T-SQL How to create function that compares string, checks difference, and do special function - sql

Question

First - sorry for my english. Second - i'm learning t-SQL.
Goal:
I want to get difference between two strings, then check in which column is this difference. If the difference is in first column, do something, if in second column - do something else.
What I'm actually doing
Column 'messages' is a string which contains list of ID. So i am replacing all '#' with ',' and deleting last ',' what gives to me ActualID and BeforeID column. See below:
DECLARE #string VARCHAR(512);
DECLARE #string2 VARCHAR(512);
DECLARE #string3 VARCHAR(512);
SET #string = '41#42#43#44#45#46#47#48#49#50#51#52#53#54#55#56#57#58#59#';
SET #string2 = REPLACE((SELECT messages FROM USERS WHERE userid = 4), '#', ', ' )
SET #string3 = left(#string2, len(#string2) - 1);
SET #string2 = REPLACE(#string, '#', ', ' )
SET #string = left(#string2, len(#string2) - 1);
SELECT #string3 as ActualID, #string as BeforeID
So now, I want compare BeforeID with ActualID. For example:
In BeforeID we have 1, 2, 3 / In ActualID 1, 2, 3, 4
In example above 4 was added. So, if it was added I want to add it to #AddedElements.
If 4, 5, 7 were added then SELECT #AddedElements as AddedElements should return 4, 5, 7 (With comas)
But, that's not all.
If BeforeID = 1, 5, 10, 14 and ActualID = 1, 5, 14 I want, that element which is in BeforeID, but not in AcutalID will be added to #DeletedElements.
So SELECT #DeletedElements as DeletedElements should return 10
Added elements/Deleted elements should be returned once. I mean, full result what I want to Earn should be
SELECT #AddedElements as AddedElements, #DeletedElements as DeletedElements
Is it possible? If, then how to do it?

Answer 1

First of all, I have to start by saying that this is just poor design; but having said that, I've also found myself in all kinds of situations where I couldn't change the way things worked, only try to make them work better in the current configuration. Therefore, I recommend something like this:
1: Create a UDF (User-Defined Function) that can handle splitting the strings and returning them in table-formed data that you can work with:
CREATE FUNCTION [dbo].[UDF_StringDelimiter]
/*********************************************************
** Takes Parameter "LIST" and transforms it for use **
** to select individual values or ranges of values. **
** **
** EX: 'This,is,a,test' = 'This' 'Is' 'A' 'Test' **
*********************************************************/
(
#LIST VARCHAR(8000)
,#DELIMITER VARCHAR(255)
)
RETURNS #TABLE TABLE
(
[RowID] INT IDENTITY
,[Value] VARCHAR(255)
)
WITH SCHEMABINDING
AS
BEGIN
DECLARE
#LISTLENGTH AS SMALLINT
,#LISTCURSOR AS SMALLINT
,#VALUE AS VARCHAR(255)
;
SELECT
#LISTLENGTH = LEN(#LIST) - LEN(REPLACE(#LIST,#DELIMITER,'')) + 1
,#LISTCURSOR = 1
,#VALUE = ''
;
WHILE #LISTCURSOR <= #LISTLENGTH
BEGIN
INSERT INTO #TABLE (Value)
SELECT
CASE
WHEN #LISTCURSOR < #LISTLENGTH
THEN SUBSTRING(#LIST,1,PATINDEX('%' + #DELIMITER + '%',#LIST) - 1)
ELSE SUBSTRING(#LIST,1,LEN(#LIST))
END
;
SET #LIST = STUFF(#LIST,1,PATINDEX('%' + #DELIMITER + '%',#LIST),'')
;
SET #LISTCURSOR = #LISTCURSOR + 1
;
END
;
RETURN
;
END
;
2: Consider dropping the whole "Switching out commas" thing, because it's pointless - the function I've written here takes two arguments: The string itself, and the delimiter (the mini-string that separates the individual strings within the big string, in your case '#') Then you just have to do a couple of quick comparisons to find out what was added and what was deleted.
DECLARE
#AddedElements VARCHAR(255) = ''
,#DeletedElements VARCHAR(255) = ''
,#ActualID VARCHAR(255) = '41#42#43#44#45#46#47#48#49#50#51#52#53#54#55#56#57#58#59#'
,#BeforeID VARCHAR(255) = '41#42#43#44#45#46#47#48#50#51#52#53#54#55#56#57#58#59#60#'
;
SET #AddedElements = #AddedElements +
SUBSTRING(
(
SELECT ', ' + Value
FROM dbo.UDF_StringDelimiter(#ActualID,'#')
WHERE Value NOT IN
(
SELECT Value
FROM dbo.UDF_StringDelimiter(#BeforeID,'#')
)
GROUP BY ', ' + Value
FOR XML PATH('')
)
,3,255)
;
SET #DeletedElements = #DeletedElements +
SUBSTRING(
(
SELECT ', ' + Value
FROM dbo.UDF_StringDelimiter(#BeforeID,'#')
WHERE Value NOT IN
(
SELECT Value
FROM dbo.UDF_StringDelimiter(#ActualID,'#')
)
GROUP BY ', ' + Value
FOR XML PATH('')
)
,3,255)
;
SELECT #AddedElements AS AddedElements,#DeletedElements AS DeletedElements
;
Using this method, if you add a value to #ActualID that does not exist in #BeforeID, it will show up in #AddedElements.
Likewise , if you remove an element from #ActualID that had previously existed in #BeforeID, it will show up in #DeletedElements.
All of this is, of course, assuming that the dynamic string (the one really being compared here) is the #ActualID. I operated with the understanding that #BeforeID is actually a stored value in the DB, and #ActualID is a dynamic string being passed in from...somewhere. If this is wrong, update me and I'll change the tactic appropriately.
Quick note: It's important to me to point out that this is just one way of dealing with a situation like this, and I'm sure there are better ways; but with the information I have, it's the best I could come up with without spending too much time and energy on it.