mysql> select nr, if(email is NULL, 'nix drin', email) as email from mailbox;
+----+----------+
| nr | email    |
+----+----------+
|  1 | nix drin |
|  2 | nix drin |
|  3 | nix drin |
|  4 | nix drin |
|  5 | nix drin |
|  6 | nix drin |
|  7 | nix drin |
|  8 | nix drin |
|  9 | nix drin |
| 10 | nix drin |
| 11 | nix drin |
| 12 | nix drin |
| 13 | nix drin |
| 14 | nix drin |
+----+----------+
14 rows in set (0.00 sec)