【MongoDB】The high query operation of MongoDB（三），mongodboperation
1, Sort
Needless to say, mongodb also support to sort data. THe positive one means sorting by asc order and negetive means by desc.
2. Group
The grammar of group:
Description Of Argument:
key： The field need to group
cond: query condition
Initial: initialize the counter of group
Redue: generally statistic operation
Finalize: further operation after group, for exampe calcuate the average.
3.
3. Distinct
it's the same as the distinct in the relational database.
4. page
The mongodb supports to use skip and limit to query by page.
5. Cursors and Cursor Methods
6. $query/$orderby/$explain
如果服务器是作为前台进程运行在终端的，就直接按Ctrl-C。否则，就用kill这种命令发出信号。如果mongod的PID是10014，就可以用kill -2 10014（SIGINT）或者kill 10014（SIGTERM）。
当mongod收到SIGINT或者SIGTERM时，会稳妥退出。也就是说会等到当前运行的操作或文件预分配完成（需要一些时间），关闭所有打开的连接，将缓存的数据刷新到磁盘，最后停止。 千万不要向运行中的MongoDB发送SIGKILL（kill -9）。这样会导致数据库直接关闭，上面降到的步骤都将被忽略，这会使数据文件损毁。
是按文件空间存储顺序存储