Introduction
String manipulation is a cornerstone of database development, whether you’re formatting output, parsing user input, or reorganizing data for reporting. Oracle SUBSTR function provides a simple yet powerful way to extract parts of strings without resorting to elaborate loops or external scripting. In this article, we’ll explore best practices and patterns for using Oracle SUBSTR in your PL/SQL code to make it cleaner, more maintainable, and efficient. We’ll cover direct extraction, combining with other functions, handling edge cases, and optimizing for performance.
1. Direct Substring Extraction
At its simplest, SUBSTR takes a source string, a starting position, and an optional length:
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SUBSTR(source_string, start_position, substring_length)
- Positive start_position counts from the first character.
- Negative start_position counts from the end.
- substring_length defaults to “through end of string” if omitted.
Example: Extracting Invoice Prefix
In a table invoices where the invoice_id values look like INV-2025-00123:
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DECLARE
v_prefix VARCHAR2(3);
BEGIN
SELECT SUBSTR(invoice_id, 1, 3)
INTO v_prefix
FROM invoices
WHERE invoice_id = ‘INV-2025-00123’;
DBMS_OUTPUT.PUT_LINE(‘Invoice prefix: ‘ || v_prefix);
END;
This returns INV cleanly.
2. Combining SUBSTR with Other String Functions
SUBSTR often shines when combined with INSTR, LENGTH, or regular expressions.
2.1 Dynamic Token Extraction
To extract a token between delimiters, use:
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v_full := ‘apple|banana|cherry’;
v_start := INSTR(v_full, ‘|’) + 1;
v_length := INSTR(v_full, ‘|’, 1, 2) – v_start;
v_token := SUBSTR(v_full, v_start, v_length);
This yields banana.
2.2 Extracting File Extensions
From report.final.xls:
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SELECT SUBSTR(filename,
INSTR(filename, ‘.’, -1) + 1)
INTO v_ext
FROM files
WHERE file_id = 101;
The -1 tells INSTR to search from the end, so you always get the correct extension.
3. Handling Edge and Error Cases
Real-world data is messy. Guard against:
- Out-of-bounds positions: SUBSTR(‘ABC’, 5, 2) returns NULL, not an error.
- Null inputs: SUBSTR(NULL, 1, 5) returns NULL.
- Multi-byte characters: Character length may differ from byte length, test on your character set.
3.1 Defensive Coding
Wrap SUBSTR calls in NVL or conditional checks when you expect missing data:
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v_code := NVL(SUBSTR(raw_code, 1, 5), ‘N/A’);
4. Improving Readability with Named Constants
Magic numbers in your code slow down comprehension. Instead of:
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vendor_code := SUBSTR(order_number, 1, 4);
Declare:
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c_vendor_length CONSTANT PLS_INTEGER := 4;
vendor_code := SUBSTR(order_number, 1, c_vendor_length);
Now readers know exactly which part you’re extracting.
5. Looping Through Dynamic Substrings
Sometimes you need all tokens in a delimited string:
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DECLARE
v_str VARCHAR2(200) := ‘A,B,C,D’;
v_pos PLS_INTEGER := 1;
v_token VARCHAR2(50);
BEGIN
LOOP
v_token := REGEXP_SUBSTR(v_str, ‘[^,]+’, 1, v_pos);
EXIT WHEN v_token IS NULL;
DBMS_OUTPUT.PUT_LINE(‘Token ‘ || v_pos || ‘: ‘ || v_token);
v_pos := v_pos + 1;
END LOOP;
END;
Though this uses REGEXP_SUBSTR, you can replicate with SUBSTR and INSTR:
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v_start := 1;
v_next := INSTR(v_str || ‘,’, ‘,’, 1, 1);
WHILE v_next > 0 LOOP
v_token := SUBSTR(v_str, v_start, v_next – v_start);
DBMS_OUTPUT.PUT_LINE(v_token);
v_start := v_next + 1;
v_next := INSTR(v_str || ‘,’, ‘,’, 1, v_pos);
v_pos := v_pos + 1;
END LOOP;
6. Performance Tips
While Oracle SUBSTR itself is lightweight, consider:
- Minimizing repeated calls: Store intermediate results in variables if reused.
- Limiting data volume: Filter rows in WHERE before applying SUBSTR.
- Avoiding full-table scans: If substrings appear in predicates, ensure functional indexes exist or use computed columns.
7. Encapsulating in Utility Packages
For enterprise code, create a utility package:
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CREATE PACKAGE string_utils IS
FUNCTION substr_safe(src VARCHAR2, start_pos PLS_INTEGER, len PLS_INTEGER := NULL)
RETURN VARCHAR2;
END;
CREATE PACKAGE BODY string_utils IS
FUNCTION substr_safe(src VARCHAR2, start_pos PLS_INTEGER, len PLS_INTEGER)
RETURN VARCHAR2 IS
BEGIN
RETURN CASE
WHEN src IS NULL THEN NULL
WHEN start_pos > LENGTH(src) THEN NULL
ELSE
CASE
WHEN len IS NULL THEN SUBSTR(src, start_pos)
ELSE SUBSTR(src, start_pos, len)
END
END;
END;
END;
Now you can call string_utils.substr_safe(…) for consistent behavior.
Conclusion
Oracle’s SUBSTR function may seem elementary, but when applied thoughtfully, it can vastly simplify your PL/SQL string logic. By combining it with INSTR and other utilities, guarding against edge cases, using named constants, and encapsulating logic in reusable libraries, you’ll produce code that’s clean, efficient, and maintainable. Adopt these patterns to ensure your string manipulation stays rock-solid as your applications grow and evolve.
