SQL Server 2022

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-- 2022+ or Azure SQL Database/MI

DECLARE @BaseStringValues VARCHAR(MAX), @ParamValues VARCHAR(MAX), @UpdatedStringValues VARCHAR(MAX)

SET @BaseStringValues = '12W4,23FE,EU3E,GH56,56Y8,34W2,QWE4'
SET @ParamValues = 'QWE4,EU3E' -- to remove

SELECT @BaseStringValues = STRING_AGG(value, ',') 
                               WITHIN GROUP (ORDER BY ordinal)
  FROM
  (
    SELECT value, ordinal
      FROM STRING_SPLIT(@BaseStringValues, ',', 1) -- order matters
  ) AS x
  WHERE NOT EXISTS
  (
    SELECT 1 
      FROM STRING_SPLIT(@ParamValues, ',', 0) -- order doesn't matter
      WHERE value = x.value
  );

SELECT @BaseStringValues;


(No column name)


12W4,23FE,GH56,56Y8,34W2


-- 2017+
DECLARE @BaseStringValues VARCHAR(MAX), @ParamValues VARCHAR(MAX), @UpdatedStringValues VARCHAR(MAX)

SET @BaseStringValues = '12W4,23FE,EU3E,GH56,56Y8,34W2,QWE4'
SET @ParamValues = 'QWE4,EU3E' -- to remove


SET @BaseStringValues = '["' + REPLACE(
  STRING_ESCAPE(@BaseStringValues, 'json'), ',','","') + '"]';

SELECT @BaseStringValues = STRING_AGG(value, ',')
                           WITHIN GROUP (ORDER BY [key])
  FROM OPENJSON(@BaseStringValues) AS x
  WHERE NOT EXISTS 
  (
    SELECT 1 FROM STRING_SPLIT(@ParamValues, ',')
    WHERE value = x.value
  );

SELECT @BaseStringValues;


(No column name)


12W4,23FE,GH56,56Y8,34W2


-- 2016 or worse
CREATE FUNCTION [dbo].[SplitString]
    (
        @List  varchar(max),
        @Delim varchar(255)
    )
    RETURNS TABLE
    AS
        RETURN ( SELECT [Value], idx = RANK() OVER (ORDER BY n) FROM 
          ( 
            SELECT n = Number, 
              [Value] = LTRIM(RTRIM(SUBSTRING(@List, [Number],
              CHARINDEX(@Delim, @List + @Delim, [Number]) - [Number])))
            FROM (SELECT Number = ROW_NUMBER() OVER (ORDER BY name)
              FROM sys.all_objects) AS x
              WHERE Number <= LEN(@List)
              AND SUBSTRING(@Delim + @List, [Number], LEN(@Delim)) = @Delim
          ) AS y
        );
DECLARE @BaseStringValues VARCHAR(MAX), @ParamValues VARCHAR(MAX), @UpdatedStringValues VARCHAR(MAX)

SET @BaseStringValues = '12W4,23FE,EU3E,GH56,56Y8,34W2,QWE4'
SET @ParamValues = 'QWE4,EU3E' -- to remove

SELECT @BaseStringValues = STUFF((SELECT
  ',' + Value
  FROM dbo.SplitString(@BaseStringvalues, ',') AS o
  WHERE NOT EXISTS
  (
    SELECT 1 FROM dbo.SplitString(@ParamValues, ',') AS x
    WHERE x.Value = o.Value
  )
  ORDER BY idx
  FOR XML PATH, TYPE).value(N'.[1]',N'varchar(max)')
  , 1, 1, '');

SELECT @BaseStringValues;


(No column name)


12W4,23FE,GH56,56Y8,34W2