mysql 查询邻近的两条记录并合并成一条

该方法虽然可以实现，但是效率特别低

现有一个签到表，需要查出每个人的每次签到和签出时间

t_sign表结构和数据如下

id	user	type	time
1	A	in	2016-10-10 09:00:00
2	B	in	2016-10-10 09:05:00
3	B	out	2016-10-10 10:00:00
5	A	out	2016-10-10 11:27:00
6	A	in	2016-10-10 13:57:00
8	B	in	2016-10-10 16:08:00
9	A	out	2016-10-10 17:45:00
10	A	in	2016-10-10 18:01:00
11	B	out	2016-10-10 18:02:00

希望查询结果

user	in_time	out_time
A	2016-10-10 09:00:00	2016-10-10 11:27:00
A	2016-10-10 13:57:00	2016-10-10 17:45:00
A	2016-10-10 18:01:00	NULL
B	2016-10-10 09:05:00	2016-10-10 10:00:00
B	2016-10-10 16:08:00	2016-10-10 18:02:00

SQL：

 select user, time as in_time,
 (select time from t_sign where user=s.user and type='out' and time > s.time order by time limit 1) out_time
 from t_sign s where type = 'in' order by user, time