How do I get textual contents from BLOB in Oracle SQL?

BLOB Extraction - The essentials

Text storage: BLOB vs. CLOB/NCLOB

While BLOBs are versatile, CLOB/NCLOB types specifically cater to textual data. If you're dealing with text in BLOBs, compatibility of character sets becomes vital for accurate conversion.

Character set - The secret decoder ring

BLOB's contents and the resulting VARCHAR2 field must maintain encoding harmony. To achieve this, use NLS_DATABASE_PARAMETERS to keep track of your database character set.

Workflow for larger BLOBs

To handle larger BLOBs, employ a clever looping strategy by converting segments within the 4000-byte limit, thus sidestepping truncation. The DBMS_LOB.SUBSTR function is your accomplice here, allowing retrieval and concatenation of 'bite-sized' segments of your BLOB.

Advanced BLOB Extraction - The extra mile

Meeting the Long BLOB challenge

For BLOBs surpassing 32767 bytes, get creative and employ a chunky chew-and-spit approach:

DECLARE
  text_output CLOB;
  temp_varchar VARCHAR2(32767);
  pos INTEGER := 1;
/* Who ordered the Long BLOB? Coming right up! */
BEGIN
  LOOP
    temp_varchar := UTL_RAW.CAST_TO_VARCHAR2(DBMS_LOB.SUBSTR(blob_col, 32767, pos));
    EXIT WHEN temp_varchar IS NULL;
    text_output := text_output || temp_varchar;
    pos := pos + LENGTH(temp_varchar);
  END LOOP;
  DBMS_OUTPUT.PUT_LINE(text_output); /* Ah, the sweet taste of BLOB text */
END;
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GUI for BLOB Visualization

For technophobes among us, friendly tools like SQL Developer or Eclipse Data Tools offer a BLOB visualization feature, serving up your BLOB's text on a silver platter.

Oracle 19c - TO_CLOB, the BLOB whisperer

In Oracle 19c or later, the TO_CLOB function is at your service. This function gracefully converts BLOBs into human-readable text, without the chore of substring operations.

Practical Techniques & Troubleshooting

Quick Peek into the BLOB

For a fast sneak peek at your BLOB's contents, whip up this select query:

/* BLOB, do not resist! Show yourself! */
SELECT DBMS_LOB.SUBSTR(blob_field, 2000, 1) AS sample_text 
FROM your_table
WHERE ROWNUM = 1;
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Frequently Faced Roadblocks

Character set mismatch - Lost in translation

If the output resembles alien handwriting (mostly '?'), it's a classic case of character set mismatch. Check your BLOB's and Oracle environment's character compatibility via NLS_LANG.

Long BLOBs - The python problem

When converting BLOB to VARCHAR2, Oracle SQL may choke on large data due to string length restrictions. A sensible chunk-wise approach mitigates this.

Binary BLOBs - Bad eggs

Converting a BLOB with binary data (images, binary files, etc.) into text leads to gobbledygook results. Always verify your BLOB's content type before laying it under the extraction scalpel.