Concatenar valores de múltiples columnas en Oracle

SQL Fiddle

Configuración del esquema de Oracle 11g R2 :

CREATE TABLE test (col1, col2, col3, col4, col5 ) AS
          SELECT 1, 'A', 'B', 'C', 'D' FROM DUAL
UNION ALL SELECT 2, 'A', 'B', 'C', NULL FROM DUAL
UNION ALL SELECT 3, 'C', 'A', NULL, NULL FROM DUAL
UNION ALL SELECT 4, 'D', 'A', 'C', NULL FROM DUAL
UNION ALL SELECT 5, NULL, NULL, NULL, NULL FROM DUAL
UNION ALL SELECT 6, NULL, NULL, NULL, 'A' FROM DUAL
UNION ALL SELECT 7, 'B', NULL, NULL, 'A' FROM DUAL
UNION ALL SELECT 8, NULL, 'C', NULL, 'A' FROM DUAL;

Consulta 1 :

Si no hay NULL valores entre otros valores (introducirá varias comas en las filas 7 y 8):

SELECT col1,
       TRIM( ',' FROM col2||','||col3||','||col4||','||col5 ) AS col2
FROM   test

Resultados :

| COL1 |    COL2 |
|------|---------|
|    1 | A,B,C,D |
|    2 |   A,B,C |
|    3 |     C,A |
|    4 |   D,A,C |
|    5 |  (null) |
|    6 |       A |
|    7 |   B,,,A |
|    8 |    C,,A |

Las dos últimas consultas funcionarán para todos los ejemplos:

Consulta 2 :

SELECT col1,
       TRIM( ',' FROM col2 || NVL2( col3, ','||col3, NULL ) || NVL2( col4, ','||col4, NULL ) || NVL2( col5, ','||col5, NULL ) ) AS col2
FROM   test

Resultados :

| COL1 |    COL2 |
|------|---------|
|    1 | A,B,C,D |
|    2 |   A,B,C |
|    3 |     C,A |
|    4 |   D,A,C |
|    5 |  (null) |
|    6 |       A |
|    7 |     B,A |
|    8 |     C,A |

Consulta 3 :

SELECT col1,
       REGEXP_REPLACE( col2||','||col3||','||col4||','||col5, '(^|,),+|,+($)', '\1' ) AS col2
FROM   test

Resultados :

| COL1 |    COL2 |
|------|---------|
|    1 | A,B,C,D |
|    2 |   A,B,C |
|    3 |    C,A, |
|    4 |   D,A,C |
|    5 |  (null) |
|    6 |       A |
|    7 |     B,A |
|    8 |     C,A |