mongodb distinct 的使用，以及如何计算distinct后的count值

db.runCommand({"distinct":"accesslog","key":"deviceId","query":{"time":/^2011-12-15/}}).values.length(); 

相当于sql: 

select count(deviceId) from (select distinct deviceId  from accesslog where time like "2011-12-15%") ; 

解释： 

查询2011-12-15日，deviceId不同的总数。 


"distinct":查询的表或collection; 
"key":根据哪个字段排重 
"query":查询条件，如果要用类似于sql中的like，则字段名字后的值要用正则匹配，例如/^2011-12-15/ 相当于 like "2011-12-15%" 

db.runCommand({"distinct":"accesslog","key":"deviceId"}) 

结果： 

{ 
        "values" : [ 
                "357194042155846", 
                "9530283B-3FCE-4A07-B939-8E03B5AD17DF", 
                "D7D2C00D-A2ED-4E64-B9B7-C2BEB7CE863E", 
        ], 
        "stats" : { 
                "n" : 323, 
                "nscanned" : 3223, 
                "nscannedObjects" : 3223, 
                "timems" : 5 
        }, 
        "ok" : 1 
} 

db.runCommand({"distinct":"accesslog","key":"deviceId","query":{"time":/^2011-12-15/}}).values.length(); 

结果： 
3