magento Add Coupn Code at sales order grid

I want to add coupon code in sales order grid . As we know Sales order gird build from sales_flat_order_grid table

Coupon_code column is exist in sales_flat_order table not sales_flat_order_grid table.

Whe I add coupon_code at sales order grid but when I search the coupon code with status column then got this error : Column 'status' in where clause is ambiguous

Note : I am working on existing project not new project

Magento version : 1.7.02 Can you please provid me solution

Thanks