要实现mysql中的sql语句，具体如下：
select max(_id),dept from test1 group by dept;
1.在mongodb中写入数据
db.test1.insert({"dept": "A", "item": { "sku": "111", "color": "red" }, "sizes": [ "S", "M" ] })
db.test1.insert({"dept": "A", "item": { "sku": "111", "color": "blue" }, "sizes": [ "M", "L" ] })
db.test1.insert({"dept": "B", "item": { "sku": "222", "color": "blue" }, "sizes": "S" })
2.mongodb中有distinct，但是只能实现如下效果
db.inventory.distinct("dept")
去重后统计数量：
db.inventory.distinct("dept").length
3.此时只能利用aggregate来完成
db.test1.aggregate(
{
$group:{_id:"$dept",minvalue:{$min:"$_id"}}
}
)
如果要实现
select max(_id),dept from test1 where xx=? group by dept;
可以增加 $match:{xx:’chenzong’} 到aggregate中去
4.如果需要实现以下sql
select * from test1 where _id in (
select max(_id) from test1group by dept
);
此时可以通过以下2步来完成：
(1).创建分组去重后的视图View_test1
db.createView("View_test1","test1",[{$group:{_id:"$dept",minvalue:{$min:"$_id"}}}])
(2).利用mongodb的$lookup来完成
db.View_test1.aggregate([{
$lookup:{
from:"test1",
localField:"minvalue",
foreignField:"_id",
as:"aa_docs"
}
}])
5.如果需要实现以下sql（去重后统计数量）
select count(1)
from (select fromUser, count(1)
from ldmError
where hosCode = '00002'
group by fromUser);
对应的mongodb语句为：
db.ldmError.group({
key:{fromUser:1},
cond:{hosCode:"00002"},
reduce:function(curr,result){
result.total++;
},
initial:{total:0}
}).length
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