i want to compare it a column has a certain string value, if yes sum the value of another column if not doesn sum anything.
how can i do that?
You want a cell to summary a whole column? That seems odd, since you will end up repeating the same sum in every row.
But anyway, you can use the 'IF' function to apply a condition.
IF ( [String Column] = "Some String", SUM([Numeric Column], 0 )