nearest as a first date,

i have a sql query for sales report,

eg simple

local ldfirst,ldend,lcfishcode1,lcfischcode2

ldfirst=thisform.txtd1.value
ldend=thisform.txtd2.value
lcfischcode1=[001] && sales fische type
lcfischcode2=[002] && sales fische type

select sum(quantity)*sum(price)/sum(quantity) as navgpriceS where fischetype= m.lcfishcode1 and ;
between(ldate,m.ldfirst,m.ldend) into cursor CrsSales

select sum(quantity)*sum(price)/sum(quantity) as navgpriceB where fischetype= m.lcfishcode2 and ;
between(ldate,m.ldfirst,m.ldend) into cursor CrsBuyings

and then i am joining above cursors with one cursor via left join,
here is my need sometimes may be there is no buyings between which is given dates at this moment
i want to use nearest buying price instead of navgpriceB at 3rd SQL

hope that i've explain :)

TIA