以下是关于sql语句的问题，求详细解释 ！

Question

'select case when (select nvl(sum(case factorcode when ''D001'' then factorvalue end),0) 
from llotherfactor where feeitemtype=''D'' and clmno=''?CaseNo?'')>0 
then least(?je_gf?,?je_gf?+(select nvl(sum(case factorcode when ''D002'' then factorvalue end),0)
from llotherfactor where feeitemtype=''D'' and clmno=''?CaseNo?''), ?amnt?-?compensatedutysumpay?) 
else least((?je_gf?+(select nvl(sum(case factorcode when ''D002'' then factorvalue end),0) 
from llotherfactor where feeitemtype=''D'' and clmno=''?CaseNo?''))*0.8,?je_gf?,?amnt?-?compensatedutysumpay?) 
end from ldsysvar where sysvar = ''onerow'''
这个查出来是一个值