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insert, update CLOB

Posted on 2004-10-11
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Last Modified: 2011-08-18
Looking for a code example (pl/sql and SQL query ...) to read, update and insert clob column types (external and internal)
with Oracle9i.
I've heard it was simplified in Oracle 9i compare to 8i where empty_clob and pointers (start and end location) should be used
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Question by:smena
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by:seazodiac
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Here is another good example:

You can initialize the LOBs by using the following SQL INSERT statement:

INSERT INTO lob_table VALUES (1001, EMPTY_BLOB(), NULL,
    EMPTY_CLOB(), NULL);

External LOBs:

INSERT INTO lob_table VALUES
         (21, NULL, NULL, NULL, BFILENAME('IMG', 'image1.gif'));
    INSERT INTO lob_table VALUES
         (22, NULL, NULL, NULL, BFILENAME('IMG', 'image2.gif'));


UPDATE lob_table SET f_lob = BFILENAME('IMG', 'image3.gif')
        WHERE key_value = 22;

BFILENAME() is a built-in function that is used to initialize the BFILE column to point to the external file.


Selecting a LOB:

DECLARE    image1       BLOB;    image_no     INTEGER := 101;BEGIN
    SELECT b_lob INTO image1 FROM lob_table        WHERE key_value = image_no;
    DBMS_OUTPUT.PUT_LINE('Size of the Image is: ' ||
        DBMS_LOB.GETLENGTH(image1));    -- more LOB routinesEND;


Example of a Read Consistent Locator

INSERT INTO lob_table
   VALUES (1, NULL, 'abcd', NULL, NULL);

COMMIT;

DECLARE
  num_var           INTEGER;
  clob_selected     CLOB;
  clob_updated      CLOB;
  clob_copied       CLOB;
  read_amount       INTEGER;
  read_offset       INTEGER;
  write_amount      INTEGER;
  write_offset      INTEGER;
  buffer            VARCHAR2(20);
 
BEGIN
  -- At time t1:
  SELECT c_lob INTO clob_selected
     FROM lob_table
     WHERE key_value = 1;

  -- At time t2:
  SELECT c_lob INTO clob_updated
     FROM lob_table
     WHERE key_value = 1
     FOR UPDATE;
 
  -- At time t3:
  clob_copied := clob_selected;
  -- After the assignment, both the clob_copied and the
  -- clob_selected have the same snapshot as of the point in time
  -- of the SELECT into clob_selected

  -- Reading from the clob_selected and the clob_copied will  
  -- return the same LOB value. clob_updated also sees the same    
  -- LOB value as of its select:
  read_amount := 10;
  read_offset := 1;  
  dbms_lob.read(clob_selected, read_amount, read_offset,
       buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'
 
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'
 
  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcd'
 
  -- At time t4:
  write_amount := 3;
  write_offset := 5;
  buffer := 'efg';
  dbms_lob.write(clob_updated, write_amount, write_offset,
       buffer);
 
  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcdefg'
 
  -- At time t5:
  read_amount := 10;
  dbms_lob.read(clob_selected, read_amount, read_offset,
       buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'
 
  -- At time t6:
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'
END;
/

Example of an Updated LOB Locator
INSERT INTO lob_table
   VALUES (1, NULL, 'abcd', NULL, NULL);

COMMIT;
 
DECLARE
  num_var          INTEGER;
  clob_updated     CLOB;
  clob_copied      CLOB;
  read_amount      INTEGER; ;
  read_offset      INTEGER;
  write_amount     INTEGER;
  write_offset     INTEGER;
  buffer           VARCHAR2(20);
BEGIN
 
-- At time t1:
  SELECT c_lob INTO clob_updated FROM lob_table
      WHERE key_value = 1
      FOR UPDATE;
 
  -- At time t2:
  clob_copied := clob_updated;
  -- after the assign, clob_copied and clob_updated see the same
  -- LOB value
 
  read_amount := 10;
  read_offset := 1;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcd'
 
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'
 

  -- At time t3:
  write_amount := 3;
  write_offset := 5;
  buffer := 'efg';
  dbms_lob.write(clob_updated, write_amount, write_offset,
        buffer);
 
  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcdefg'
 

  -- At time t4:
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'
 

  -- At time t5:
  clob_copied := clob_updated;
 
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcdefg'
END;
/

Copying internal LOBs

INSERT INTO lob_table1 (key_value, b_lob)
    (SELECT key_value, b_lob FROM lob_table2 T2
     WHERE T2.key_value = 101);

creates a new LOB locator in the table lob_table1, and copies the LOB data from lob_table2 to the location pointed to by a new LOB locator which is inserted into table lob_table1.


----------

LOB Reference

EMPTY_BLOB() and EMPTY_CLOB() Functions

INSERT INTO lob_table VALUES (1001, EMPTY_BLOB(), 'abcde', NULL, NULL);
UPDATE lob_table SET c_lob = EMPTY_CLOB() WHERE key_value = 1001;
INSERT INTO lob_table VALUES (1002, NULL, NULL, NULL, NULL);

BFILENAME() Function

DBMS_LOB Package
The DBMS_LOB package provides routines to access BLOBs, CLOBs, NCLOBs, and BFILEs. You can use DBMS_LOB for access and manipulation of specific parts of a LOB, as well as complete LOBs. DBMS_LOB can read as well as modify BLOBs, CLOBs, and NCLOBs, and provides read-only operations on BFILEs.

All DBMS_LOB routines work based on LOB locators. For the successful completion of DBMS_LOB routines, you must provide an input locator that represents a LOB that already exists in the database tablespaces or external filesystem.


The routines that can modify BLOB, CLOB, and NCLOB values are:

APPEND() - append the contents of the source LOB to the destination LOB
COPY() - copy all or part of the source LOB to the destination LOB
ERASE() - erase all or part of a LOB
LOADFROMFILE() - load BFILE data into an internal LOB
TRIM() - trim the LOB value to the specified shorter length
WRITE()- write data to the LOB from a specified offset
The routines that read or examine LOB values are:

GETLENGTH() - get the length of the LOB value
INSTR() - return the matching position of the nth occurrence of the pattern in the LOB
READ() - read data from the LOB starting at the specified offset
SUBSTR() - return part of the LOB value starting at the specified offset
The read-only routines specific to BFILEs are:

FILECLOSE() - close the file
FILECLOSEALL()- close all previously opened files
FILEEXISTS() - check if the file exists on the server
FILEGETNAME() - get the directory alias and file name
FILEISOPEN() - check if the file was opened using the input BFILE
locators
FILEOPEN() - open a file
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