-- твой код - первая таблица
mysql> CREATE TABLE `user` (
    ->  `login` VARCHAR(100) NULL DEFAULT NULL,
    ->  `inUse` INT(11) NULL DEFAULT NULL
    -> )
    -> COLLATE='utf8_general_ci'
    -> ENGINE=InnoDB
    -> ;
Query OK, 0 rows affected (2.26 sec)

-- твой код - вторая таблица
mysql> CREATE TEMPORARY TABLE `usertest` (
    ->  `lo` varchar(100) DEFAULT NULL
    -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;
Query OK, 0 rows affected (0.27 sec)

-- произвольные значения
mysql> INSERT INTO user (login, inUse)
    -> SELECT 'user1',1 UNION ALL
    -> SELECT 'user2',0 UNION ALL
    -> SELECT 'user3',0 UNION ALL
    -> SELECT 'user4',1 ;
Query OK, 4 rows affected (0.28 sec)
Records: 4  Duplicates: 0  Warnings: 0

-- значения из первого постинга
mysql> INSERT INTO usertest (lo)
    -> SELECT 'user1' UNION ALL
    -> SELECT 'user2' ;
Query OK, 2 rows affected (0.02 sec)
Records: 2  Duplicates: 0  Warnings: 0

-- твой запрос
mysql> select *
    -> from user u
    -> where u.login in (select t.lo from userTest t);
+-------+-------+
| login | inUse |
+-------+-------+
| user1 |     1 |
| user2 |     0 |
+-------+-------+
2 rows in set (0.03 sec)