Oracle 11g Release 2

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CREATE TABLE a ( x ) AS
SELECT 10 FROM DUAL UNION ALL
SELECT 10 FROM DUAL UNION ALL
SELECT 10 FROM DUAL;
3 rows affected

SELECT x
FROM   A
WHERE  x = 10
UNION ALL
SELECT NULL
FROM   DUAL
WHERE  NOT EXISTS (
  SELECT x
  FROM   A
  WHERE  x = 10
)


X


10

10

10


DELETE FROM a;
3 rows affected

SELECT x
FROM   A
WHERE  x = 10
UNION ALL
SELECT NULL
FROM   DUAL
WHERE  NOT EXISTS (
  SELECT x
  FROM   A
  WHERE  x = 10
)


X


null