Tonumber function Error

Hi Every one,

I have a column called ATV in my sql database.It is a string type.i want to apply rounding for this in report level only.For that I am trying to convert this to number using to number function.After converting if i place this formula filed in my report it is giving error like

The string is non numeric

I need to apply rounding for the field, please suggest me an approach.

The formula written for this field in database is

(SUM(c.sale)/SUM(c.transaction_count)),'-') AS ATV

Here some times transaction_count is zero