Translation of sql query to Mongo languague

问题

I am trying to do the following query in Mongo:

SELECT * 
FROM events 
WHERE v1 - v2 > 20;

where v1 and v2 are names of columns in the SQL DB.

how can I translate it to Mongo Languague?

回答1:

Assume you have these documents in your events:

{
    "_id" : ObjectId("54cdfde528bb923955eff8b4"),
    "v1" : 35,
    "v2" : 10
}

/* 1 */
{
    "_id" : ObjectId("54cdfde928bb923955eff8b5"),
    "v1" : 10,
    "v2" : 20
}

You cant use a simple find() query. You should use aggregation:

db.events.aggregate(
[    
    { $project : { 'diff' : { $subtract : ['$v1', '$v2'] }, v1 : 1, v2 : 1 } },
    { $match : { diff : { $gt : 20} } },
    { $project : { v1 : 1, v2 : 1 } },
]).result

the result is:

{
    "0" : {
        "_id" : ObjectId("54cdfde528bb923955eff8b4"),
        "v1" : 35,
        "v2" : 10
    }
}

来源：https://stackoverflow.com/questions/28261506/translation-of-sql-query-to-mongo-languague