while loop inside a trigger to loop through all the columns of table in sql

Question

I have a trigger like below on user table to insert into the audit table with which column was updated and previous value:

ALTER TRIGGER [dbo].[trgAfterUpdat         


        

Answer 1

Try using query similar to this one – it will generate If statements for all columns of a given table.

Note: This is not fully tested and probably needs more adjustments but you see the idea behind it.

select 'if update(' + C.name + ')
  begin
      select @OldValue=j.' + C.name + ' from deleted j;  
      set @fieldname = ''' + C.name + ''';

        insert into tbl_Audit(user_key, field_name, previuos_Value, user_name)
        values(@User_Key ,@fieldname,@OldValue, @CreateUser);
  end'
from sys.all_columns C
inner join sys.tables T on C.object_id = T.object_id
where T.name = 'table_name' and T.schema_id = SCHEMA_ID('schema_name')

If wouldn’t go with while loop because it can probably cause perf issues…

Answer 2

Try this one -

ALTER TRIGGER [dbo].[trgAfterUpdate] 

    ON [dbo].[tbl_User]
    AFTER UPDATE

AS BEGIN

    SET NOCOUNT ON
    SET XACT_ABORT ON

    DECLARE @DocumentUID UNIQUEIDENTIFIER

    DECLARE cur CURSOR FORWARD_ONLY READ_ONLY LOCAL FOR
        SELECT DocumentUID, ...
        FROM INSERTED

    OPEN cur

    FETCH NEXT FROM cur INTO @DocumentUID, ...

    WHILE @@FETCH_STATUS = 0 BEGIN

        DECLARE 
              @BeforeChange XML 
            , @AfterChange XML

        SELECT @BeforeChange = (
            SELECT *
            FROM DELETED
            WHERE [DocumentUID] = @DocumentUID
            FOR XML RAW, ROOT
        )
        , @AfterChange = (
            SELECT *
            FROM INSERTED
            WHERE [DocumentUID] = @DocumentUID
            FOR XML RAW, ROOT
        )

        INSERT INTO dbo.LogUser (DocumentUID, BeforeChange, AfterChange)
        SELECT @DocumentUID, @BeforeChange, @AfterChange

        -- your business logic 

        FETCH NEXT FROM cur INTO @DocumentUID, ...

    END

    CLOSE cur
    DEALLOCATE cur

END