Human-Readable PL/SQL: Transforming Query Output with Oracle LISTAGG

Introduction

Data consumers, business analysts, managers, auditors, rarely want to sift through dozens of rows to find a concise summary. They prefer one-line summaries that read like natural language: “User 123 listened to Blues, Jazz, and Rock” or “Order 456 includes Widget A, Widget B, and Widget C.” Oracle LISTAGG delivers exactly that in SQL, and when combined with PL/SQL, it becomes a powerful tool to transform raw query output into human-readable strings. In this article, we’ll explore how to craft readable output using LISTAGG, handle edge cases, integrate it into PL/SQL procedures, and follow best practices so your reports read like well-written prose rather than machine dumps.

1. Crafting Clear Output with Proper Delimiters

While a comma and space (‘, ‘) is the most common delimiter, thoughtful choices can improve clarity:

• Oxford comma style: When listing multiple items, add an “and” before the last element.
  
• Custom separators: Use semicolons (‘; ‘) if values themselves contain commas (e.g., “Paris, TX”).
  

Example: Building an Oxford comma list in PL/SQL

plsql

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DECLARE

  v_raw_list VARCHAR2(4000);

  v_items    SYS.ODCIVARCHAR2LIST;  — PL/SQL collection type

  v_readable VARCHAR2(4000);

BEGIN

  SELECT LISTAGG(tag_name, ‘, ‘)

           WITHIN GROUP (ORDER BY tag_name)

    INTO v_raw_list

  FROM article_tags

  WHERE article_id = 100;

  v_items := SYS.ODCIVARCHAR2LIST();  

  FOR i IN 1 .. REGEXP_COUNT(v_raw_list, ‘, ‘) + 1 LOOP

    v_items.EXTEND;

    v_items(i) := REGEXP_SUBSTR(v_raw_list, ‘[^,]+’, 1, i);

  END LOOP;

  IF v_items.COUNT = 1 THEN

    v_readable := v_items(1);

  ELSIF v_items.COUNT = 2 THEN

    v_readable := v_items(1) || ‘ and ‘ || v_items(2);

  ELSE

    v_readable := SUBSTR(v_raw_list, 1, INSTR(v_raw_list, ‘,’, -1) – 1)

                  || ‘, and ‘

                  || v_items(v_items.COUNT);

  END IF;

  DBMS_OUTPUT.PUT_LINE(‘Tags: ‘ || v_readable);

END;

This turns “Alpha, Beta, Gamma” into “Alpha, Beta, and Gamma,” making output more natural.

2. Handling Singular vs. Plural Labels

Human-readable output also adapts labels based on count:

plsql

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DECLARE

  v_list  VARCHAR2(4000);

  v_count NUMBER;

  v_label VARCHAR2(20);

BEGIN

  SELECT COUNT(*),

         LISTAGG(error_code, ‘, ‘)

           WITHIN GROUP (ORDER BY error_code)

    INTO v_count, v_list

    FROM process_errors

   WHERE run_id = 789;

  v_label := CASE WHEN v_count = 1 THEN ‘error’ ELSE ‘errors’ END;

  DBMS_OUTPUT.PUT_LINE(v_count || ‘ ‘ || v_label || ‘: ‘ || v_list);

END;

This outputs “1 error: X123” or “3 errors: X123, Y456, Z789” as appropriate.

3. Embedding LISTAGG in Stored Functions

To reuse logic, encapsulate it in a PL/SQL function:

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CREATE OR REPLACE FUNCTION get_user_genres(p_user_id IN NUMBER)

  RETURN VARCHAR2 IS

  v_genres VARCHAR2(4000);

BEGIN

  SELECT LISTAGG(genre, ‘, ‘)

           WITHIN GROUP (ORDER BY genre)

    INTO v_genres

    FROM user_listens

   WHERE user_id = p_user_id;

  RETURN v_genres;

END;

Clients can then:

sql

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SELECT user_id,

       get_user_genres(user_id) AS genres

  FROM users;

4. Combining LISTAGG with Other Text Functions

Often, you need to enrich the aggregated list with context:

plsql

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DECLARE

  v_genres VARCHAR2(4000);

  v_msg    VARCHAR2(4050);

BEGIN

  v_genres := get_user_genres(42);

  IF v_genres IS NULL THEN

    v_msg := ‘User 42 has not listened to any genres yet.’;

  ELSE

    v_msg := ‘User 42 has listened to: ‘ || v_genres || ‘.’;

  END IF;

  DBMS_OUTPUT.PUT_LINE(v_msg);

END;

This provides clear, sentence-like feedback.

5. Avoiding Common Pitfalls

• Null handling: LISTAGG ignores NULLs, but concatenation with NULL strings can produce extra separators. Always trim or NVL results.
  
• Length limits: Remember the 4000/32767 limit and use ON OVERFLOW TRUNCATE or CLOB casts.
  
• Ordering: Use ORDER BY within LISTAGG to ensure predictable output.
  

Conclusion

By harnessing Oracle LISTAGG within PL/SQL, you can transform dry, row-based query results into polished, human-readable summaries that feel like natural language. Whether you’re creating one-line audit logs, user-friendly dashboards, or automated reports, combining LISTAGG with PL/SQL functions, control-flow logic, and text manipulation yields output that speaks directly to business users. Follow best practices, choose appropriate delimiters, handle singular vs. plural, manage empty or overflow cases, and encapsulate logic in reusable functions, to make your database deliver not just data, but clear, readable stories.