Retreive only cheapest one from duplicates items

[BenOussama180]

is there a way to select distinct or unique values from a table based on condition ?
for example i have table of links, each link got a price and site columns
i want to retreive only the cheapest link ( lower price ) from duplicates links ( links with the same site ) !
and it should be a DATASOURCE so powergrid can read it !

I tried this

but it throws this error