DataSource Controls :: Sql Query Syntax For Advanced Search With Dropdownlists

I want to be able to make a query from three dropdownlists but not all three have to be selected to create results. This is what I have so far :

sub getdata (src as object, e as eventargs)
dim strsql as string
dim myconnection as oledbconnection
dim mycommand as oledbcommand
myconnection = new oledbconnection(ConfigurationSettings.AppSettings("MM_CONNECTION_STRING_csrdb"))
strsql ="SELECT tbl_case.Case_id, tbl_case.Title, tbl_case.Description, Tbl_Country.Country_Name, tbl_principle.Principle_Name," _
& "tbl_keyword.Keyword FROM tbl_keyword INNER JOIN ((tbl_principle INNER JOIN (Tbl_Country INNER JOIN tbl_case ON Tbl_Country.Country_Id
tbl_case.country) ON tbl_principle.Principle_ID = tbl_case.Principle) INNER JOIN tbl_Usedkeywords ON tbl_case.Case_id
= tbl_Usedkeywords.case) ON tbl_keyword.Keyword_id = tbl_Usedkeywords.keyword " _
& "WHERE "
if DLPrinciple.selectedindex=0 then
strsql = strsql & "((tbl_principle.Principle_Name) like '%')"
else
if DLCountry.Selectedindex=0 then
strsql = strsql & "((tbl_country.country_Name) like '%')"
else
if DLKeyword.selectedindex=0 then
strsql = strsql & "((tbl_keyword.Keyword) like '%')"
else
strsql = strsql & "((tbl_principle.principle_Name) = '" & session("infoprinciple") & "')" + "((tbl_country.country_Name) = '" & session("infoland") & "')" + "((tbl_keyword.keyword) = '" & session("infokeyword") & "')"
end if
end if
end if

This is working perfectly with only one dropdownlist but I can not figure out how to make it work with several.