sql order by

SELECT x, y
FROM (SELECT 1 AS x, true AS y UNION ALL
      SELECT 9, true UNION ALL
      SELECT NULL, false)
ORDER BY x;
+------+-------+
| x    | y     |
+------+-------+
| NULL | false |
| 1    | true  |
| 9    | true  |
+------+-------+

SELECT x, y
FROM (SELECT 1 AS x, true AS y UNION ALL
      SELECT 9, true UNION ALL
      SELECT NULL, false)
ORDER BY x NULLS LAST;
+------+-------+
| x    | y     |
+------+-------+
| 1    | true  |
| 9    | true  |
| NULL | false |
+------+-------+

SELECT x, y
FROM (SELECT 1 AS x, true AS y UNION ALL
      SELECT 9, true UNION ALL
      SELECT NULL, false)
ORDER BY x DESC;
+------+-------+
| x    | y     |
+------+-------+
| 9    | true  |
| 1    | true  |
| NULL | false |
+------+-------+

SELECT x, y
FROM (SELECT 1 AS x, true AS y UNION ALL
      SELECT 9, true UNION ALL
      SELECT NULL, false)
ORDER BY x DESC NULLS FIRST;
+------+-------+
| x    | y     |
+------+-------+
| NULL | false |
| 9    | true  |
| 1    | true  |
+------+-------+