Oracle TO_CLOB (bfile|blob) Function: A Simple Guide

The TO_CLOB (bfile|blob) function in Oracle SQL is a conversion function. Its job is to take binary data, either from an external file (BFILE) or an internal binary object (BLOB), and convert it into a CLOB (Character Large Object).

This function is essential when you have data that is text but is stored in a binary format. For example, you might have a text file or an XML document stored in a BLOB or saved on the server's file system as a BFILE. TO_CLOB allows you to read this binary data as text.

What is the TO_CLOB (bfile|blob) Function in Oracle?

The TO_CLOB(binary_value, [csid], [mime_type]) function reads a BFILE or BLOB and returns its contents as a CLOB, interpreted using a character set you can specify.

• Input: A BLOB (internal binary) or BFILE (external file pointer).
• Output: A CLOB (internal character object).
• csid (Character Set ID): An optional number that specifies the character set of the source BFILE or BLOB data.
• mime_type (MIME Type): An optional text string you can assign to the new CLOB.

This is different from TO_CHAR(bfile|blob) because the output is a CLOB, which can hold gigabytes of text, rather than a VARCHAR2, which is very limited in size.

TO_CLOB (bfile|blob) Function Syntax

The syntax for TO_CLOB (bfile|blob) is:

TO_CLOB(bfile_or_blob_value, [csid], [mime_type])

Let's break that down:

• bfile_or_blob_value: The BFILE or BLOB column or value you want to convert.
• [csid] (Optional): A numeric Character Set ID. For example, 873 is often used for WE8ISO8859P1. If omitted (or 0), the database character set is assumed.
• [mime_type] (Optional): A text string like 'text/xml' or 'application/json' to tag the resulting CLOB.

Note: This is an Advanced Function

Using this function with BFILEs requires database setup. A DBA must create a DIRECTORY object (e.g., CREATE DIRECTORY my_files AS '/server/docs') and you must use the BFILENAME function to get a file pointer. The examples below are hypothetical to show how the function is used.

Oracle TO_CLOB (bfile|blob) Function Examples

Here are two practical examples of how to use TO_CLOB (bfile|blob).

Example 1: Converting a BLOB to a CLOB using TO_CLOB

Imagine you have a table logs with a BLOB column log_data that stores large text log files. You want to select this BLOB data as a CLOB to search it for text.

Query:

-- This query reads the binary data from the BLOB
-- and converts it to a CLOB in the database character set.
SELECT 
  log_id,
  TO_CLOB(log_data) AS "Log_Text"
FROM 
  logs
WHERE 
  log_id = 734;

Result: (The query returns the binary log data as a fully searchable CLOB.)

Example 2: Converting an External File (BFILE) to a CLOB using TO_CLOB

This example shows how to read an external XML file (stored as a BFILE) into the database as a CLOB, while also tagging its MIME type.

(This assumes a DIRECTORY object named DOC_DIR exists.)

Query:

-- We use BFILENAME to get a pointer to the file 'report.xml'
-- We specify '873' as its character set ID
-- We tag the resulting CLOB with 'text/xml'
INSERT INTO my_clob_table (id, clob_document)
VALUES (
  101,
  TO_CLOB(
    BFILENAME('DOC_DIR', 'report.xml'),
    873,
    'text/xml'
  )
);

Result: (This statement inserts the text content of the external file report.xml into the my_clob_table as a CLOB.)