这段oracle的存储函数代码错在哪？

[wxdatbit]

create or replace function GetBookDetail(aRecNo in VARCHAR2)
return VARCHAR2
is
bookDetail VARCHAR2(1000);
Cursor Enn(myRecNo varchar2) is
select MARC_IDX_CONT from libsys.Marc_idx
where MARC_DATA_CODE in ('200i','701a','210d','210c')
and Marc_Rec_No = myRecNo;

begin
for currentPart in Enn(aRecNo) Loop
bookDetail := bookDetail || ' ' || currentPart.MARC_IDX_CONT;
end Loop;
--Close Enn;
GetBookDetail := '<a href=http://localhost/' || aRecNo || '>' || bookDetail || '</a><b>';
Return GetBookDetail;
end;

[black_dragon]

变量bookDetail没有赋初值，所以值为null
bookDetail := bookDetail || ' ' || currentPart.MARC_IDX_CONT;
null与其它字符串连接还是null