Alter all/specific columns by double dynamic SQL

USE MyDB
GO


DECLARE @sqlselect NVARCHAR(500);
DECLARE @sql NVARCHAR(500);
DECLARE @ID INT;

SELECT @sqlselect = 'select ''ALTER TABLE ''+TABLE_CATALOG+''.''+TABLE_SCHEMA+''.''+TABLE_NAME+'' ALTER COLUMN ''+COLUMN_NAME+'' ''+DATA_TYPE+'' ''+case when DATA_TYPE=''varchar''
then ''(''+LTRIM(RTRIM(cast(CHARACTER_MAXIMUM_LENGTH as char(3))))+'')''
else ''''
end+'' NULL;'' from INFORMATION_SCHEMA.COLUMNS
where TABLE_CATALOG=''MyDB'' and TABLE_SCHEMA=''Myschema''
and IS_NULLABLE=''NO''
and COLUMN_NAME not in (''MyColumn if required'')';

IF OBJECT_ID('tempdb.dbo.#tempQuery', 'U') IS NOT NULL
DROP TABLE #tempQuery;

CREATE TABLE #tempQuery (
ID INT IDENTITY(1, 1)
,abc NVARCHAR(max)
);

INSERT INTO #tempQuery
EXEC @sql = sp_executesql @sqlselect;

SELECT @ID = max(ID)
FROM #tempQuery;

WHILE (@ID > 0)
BEGIN
SELECT @sql = abc
FROM #tempQuery
WHERE ID = @ID;

EXECUTE (@sql);

SET @ID = @ID - 1;
END;

IF OBJECT_ID('tempdb.dbo.#tempQuery', 'U') IS NOT NULL
DROP TABLE #tempQuery;