if condition in sql server update query

Question

I have a SQL server table in which there are 2 columns that I want to update either of their values according to a flag sent to the stored procedure along with the new value, so

Answer 1

Something like this should work:

UPDATE
    table_Name
SET 
  column_A = CASE WHEN @flag = '1' THEN column_A + @new_value ELSE column_A END,
  column_B = CASE WHEN @flag = '0' THEN column_B + @new_value ELSE column_B END
WHERE
    ID = @ID