ON (Table1.Fld1='' AND Table1.Fld2 = Table2.Fld2 ) OR (Table1.Fld1<>'' AND Table1.Fld1 = Table2.Fld1)>
>select * from Table1 left join Table2 ON CASE WHEN LEN(RTRIM(Table1.Fld1))=0 THEN Table1.Fld2 = Table2.Fld2 > ELSE Table1.Fld1 = Table2.Fld1 END >>