Come faccio a combinare i miei primi 5 stipendi e gli ultimi 5 da visualizzare in Oracle?

Usa UNION o UNION ALL :

SELECT SAL FROM
(
SELECT DISTINCT SAL FROM EMP WHERE SAL IS NOT NULL ORDER BY SAL DESC
)
WHERE ROWNUM < 6
UNION ALL
SELECT SAL FROM
(
SELECT DISTINCT SAL FROM EMP WHERE SAL IS NOT NULL ORDER BY SAL ASC
)
WHERE ROWNUM < 6;

Puoi anche scriverlo in modo più succinto come:

SELECT SAL
FROM (SELECT DISTINCT SAL,
             DENSE_RANK() OVER (ORDER BY sal) as seqnum_asc,
             DENSE_RANK() OVER (ORDER BY sal) as seqnum_desc
      FROM EMP
      WHERE SAL IS NOT NULL
     ) s
WHERE seqnum_asc < 6 OR seqnum_desc < 6;