SELECT id, x
FROM (
  SELECT id, x
  FROM test
  WHERE x='y'
  LIMIT 1
) t1
UNION ALL
SELECT id, x
FROM (
  SELECT id, x
  FROM test
  ORDER BY id DESC
  LIMIT 5
) t2;

# Или так

(SELECT id, x
FROM test
WHERE x='y'
LIMIT 1)
UNION ALL
SELECT id, x
FROM (
  SELECT id, x
  FROM test
  ORDER BY id DESC
  LIMIT 5
) t2;

# Или вот так

(SELECT id, x
FROM test
WHERE x='y'
LIMIT 1)
UNION ALL
(
  SELECT id, x
  FROM test
  ORDER BY id DESC
  LIMIT 5
);