题目内容

下面哪个操作符不能放在aggregate的第一级？

A. $group
B. $match
C. $sort
D. $sum

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以下不能实现统计各个部门男教师的人数的命令是？

A. db.teachers.aggregate([{$match:{"sex":"男"}},{$group:{"_id":{"dept":"$dept"},"num_count":{$sum:1}}}])
B. db.teachers.aggregate([{$group:{"_id":{"dept":"$dept"},"num_count":{$sum:1}}},{$match:{"sex":"男"}}])
C. db.teachers.group({"key":{"dept":2},"initial":{"count":0},"reduce":function(cur,result){return result.count++},"cond":{"sex":"男"}})
D. db.teachers.runCommand({"group":{"ns":"teachers","key":{"dept":0},"initial":{"count":0},"$reduce": function(cur,result){result.count++;},"cond":{"sex":"男"}}})

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