下面是對lob數據類型的簡單介紹。
blob:二進制lob,為二進制數據,最長可達4GB,存貯在數據庫中。
clob:字符lob,字符數據,最長可以達到4GB,存貯在數據庫中。
bfile:二進制文件;存貯在數據庫之外的只讀型二進制數據,最大長度由操作系統限制。
nclob:支持對字節字符集合(nultibyte characterset)的壹個clob列。
對於如何檢索和操作這些lob數據壹直是oracle數據庫開發者經常碰到的問題。下面我將在oracle對lob數據處理的壹些方法和技巧,介紹給讀者,希望能夠對讀者以後的開發有所幫助。
oracle中可以用多種方法來檢索或操作lob數據。通常的處理方法是通過dbms_lob包。
其他的方法包括使用api(application programminginterfaces)應用程序接口和oci(oracle call interface)oracle調用接口程序。
壹、在oracle開發環境中我們可以用dbms_lob包來處理!dbms_lob包功能強大,簡單應用。既可以用來讀取內部的lob對象,也可以用來處理bfile對象。但處理兩者之間,還有壹點差別。處理內部lob對象(blob,clob)時,可以進行讀和寫,但處理外部lob對象bfile時,只能進行讀操作,寫的操作可以用pl/sql處理。另外用sql也可以處理lob,但要註意sql僅可以處理整個lob,不能操作lob的數據片。
在dbms_lob包中內建了read(),append,write(),erase(),copy(),getlength(),substr()等函數,可以很方便地操作lob對象。這裏不做深入討論,讀者可以參看相關的書籍。
對於pl/sql,下面介紹壹種技巧,用動態的pl/sql語句處理clob對象來傳替表名!
example 1.
動態PL/SQL,對CLOB字段操作可傳遞表名table_name,表的唯壹標誌字段名field_id,clob字段名field_name記錄號v_id,開始處理字符的位置v_pos,傳入的字符串變量v_clob
修改CLOB的PL/SQL過程:updateclob
create or replace procedure updateclob(
table_name in varchar2,
field_id in varchar2,
field_name in varchar2,v_id in number,
v_pos in number,
v_clob in varchar2)
is
lobloc clob;
c_clob varchar2(32767);
amt binary_integer;
pos binary_integer;
query_str varchar2(1000);
begin
pos:=v_pos*32766+1;
amt := length(v_clob);
c_clob:=v_clob;
query_str :='select '||field_name||'from '||table_name||'
where '||field_id||'= :id for update ';
--initialize buffer with data to be inserted or updated
EXECUTE IMMEDIATE query_str INTO lobloc USING v_id;
--from pos position, write 32766 varchar2 into lobloc
dbms_lob.write(lobloc, amt, pos, c_clob);
commit;
exception
when others then
rollback;
end;
l /用法說明:
在插入或修改以前,先把其它字段插入或修改,CLOB字段設置為空empty_clob(),
然後調用以上的過程插入大於2048到32766個字符。
如果需要插入大於32767個字符,編壹個循環即可解決問題。
查詢CLOB的PL/SQL函數:getclob
create or replace function getclob(
table_name in varchar2,
field_id in varchar2,
field_name in varchar2,
v_id in number,
v_pos in number) return varchar2
is
lobloc clob;
buffer varchar2(32767);
amount number := 2000;
offset number := 1;
query_str varchar2(1000);
begin
query_str :='select '||field_name||' from '||table_name||'
where '||field_id||'= :id ';
--initialize buffer with data to be found
EXECUTE IMMEDIATE query_str INTO lobloc USING v_id;
offset:=offset+(v_pos-1)*2000;
--read 2000 varchar2 from the buffer
dbms_lob.read(lobloc,amount,offset,buffer);
return buffer;
exception
when no_data_found then
return buffer;
end;
l 用法說明:
用select getclob(table_name,field_id,field_name,v_id,v_pos) as
partstr from dual;
可以從CLOB字段中取2000個字符到partstr中,
編壹個循環可以把partstr組合成dbms_lob.getlength(field_name)長度的目標字符串。
二、對於在其他不同的開發環境,例如vc,vb,pb,java等環境下對lob的處理,處理方法不盡相同,在這裏將簡要舉幾個例子來說明不在oracle開發環境下對lob的處理。
(壹) 在pb中的處理
exampler 2.
string ls_path,ls_filename,ls_jhdh
long ll_num,ll_count,rtn
blob ole_blob
ll_num=dw_lb.getrow()
if ll_num>0 then ls_jhdh=dw_lb.object.ct_njhdh[ll_num]
select count(*) into :ll_count from sj_jh_jhfjb where
ct_jhdlxbh='1' and ct_jhdh=:ls_jhdh and ct_jdlxbh=:is_jdlx;
if ll_count>0 then
rtn=messagebox("提示","是否要修改此附件",question!,yesno!,1)
if rtn=1 then
SELECTBLOB ct_jhfjnr INTO le_blob from sj_jh_jhfjb where
ct_jhdlxbh='1' and ct_jhdh=:ls_jhdh and ct_jdlxbh=:is_jdlx;
ole_1.objectdata =ole_blob
If ole_1.activate(offsite!) <> 0 Then
Messagebox("OLE Activate","不能激活")
Return -1
end If
end if
else
messagebox("提示","沒有附件")
end if
end if
(二)在vb中的處理
在vb中處理大對象,壹般可以用OO4O(oracle objects for
ole)來處理大對象。這裏介紹壹種不用0040處理大對象blob的方法。
下面這段程序可以將壹個文件(文本文件,doc文件,圖象文件等)保存到數據庫中,並可以將其從數據庫讀出
需要兩個commandbutton
cmd1 名稱 cmdsave caption 保存
cmd2 名稱 cmdread caption 讀取
壹個cmddialog控件
同時需要創建壹張表t_demo(字段id 類型 number,;字段text 類型 blob;)
exmple 3.
Option Explicit
Dim rn As ADODB.Connection
Public Function CreateDataSource(DataSource As String, UserID
As String, Password As String) As Boolean
On Error GoTo DbConErr:
Set rn = New ADODB.Connection
With rn
.ConnectionString = "Provider=OraOledb.Oracle.1;" & _
"password=" & Password & ";" & _
"User ID =" & UserID & ";" & _
"Data Source=" & DataSource & ";" & _
"Locale Identifier=2052"
.Open
End With
CreateDataSource = True
Exit Function
DbConErr:
CreateDataSource = False
End Function
Private Sub cmdRead_Click()
Dim rs As New ADODB.Recordset
rs.ActiveConnection = rn
rs.LockType = adLockOptimistic
rs.CursorLocation = adUseClient
rs.Source = "select * from t_demo"
rs.Open
ComDlgDir.DialogTitle = "保存文件"
ComDlgDir.Filter = "*.*"
ComDlgDir.ShowSave
Call BlobToFile(rs.Fields("text"), ComDlgDir.filename)
Set rs = Nothing
Exit Sub
Set rs = Nothing
End Sub
Private Sub cmdsave_Click()
Dim rs As New ADODB.Recordset
rs.ActiveConnection = rn
rs.LockType = adLockOptimistic
rs.CursorLocation = adUseClient
rs.Source = "select * from t_demo"
rs.Open
rs.AddNew
ComDlgDir.DialogTitle = "選取文件"
ComDlgDir.ShowOpen
rs.Fields("id").Value = 1
If ComDlgDir.filename <> "" Then
Call FileToBlob(rs.Fields("text"), ComDlgDir.filename)
rs.Update
End If
Set rs = Nothing
Exit Sub
Set rs = Nothing
End Sub
Private Sub Form_Load()
If Not CreateDataSource("sid", "systemp", "manager") Then
MsgBox "Connection failure!"
End If
End Sub
fld As ADODB.Field, filename As String, Optional ChunkSize As
Long = 8192)
Dim fnum As Integer, bytesleft As Long, bytes As Long
Dim tmp() As Byte
If (fld.Attributes And adFldLong) = 0 Then
Err.Raise 1001, , "field doesn't support the GetChunk method."
End If
If Dir$(filename) = "" Then Err.Raise 53, , "File not found"
fnum = FreeFile
Open filename For Binary As fnum
bytesleft = LOF(fnum)
Do While bytesleft
bytes = bytesleft
If bytes > ChunkSize Then bytes = ChunkSize
ReDim tmp(1 To bytes) As Byte
Get fnum, , tmp
fld.AppendChunk tmp
bytesleft = bytesleft - bytes
Loop
Close #fnum
End Sub
Sub BlobToFile(fld As ADODB.Field, filename A