Need to convert this SQL query to MongoDB

Question

I am new to MongoDB. I need to convert this SQL code to MongoDB

select TOP 5 r.regionName, COUNT(c.RegionID)
from region as r,
     company as c
where c.RegionID = r._id
group by r.regionName
order by COUNT(c.RegionID) DESC;

Answer 1

Option 1. You can use the aggregation framework with $lookup , $group , $project , $sort and $limit stages, but this seems like a wrong approach since the true power to change relation database with mongoDB is the denormalization and avoidance of join ( $lookup ) like queries.

Option 2. You convert your multi-table relational database schema to document model and proceed with simple $group , $project , $sort and $limit stage aggregation query for the above task.

Since you have not provided any mongodb document examples it is hard to provide how your queries will look like...

Answer 2

You should show attempt of converting it, as indicated in the comments.

You could try using an online converting tool, like this SQL Query to MongoDB Converter , it should work with simple queries. For your query, it returns:

db.regionas r, company as c .group({
   "key":{
         "r.regionName order by COUNT(c.RegionID) DESC": true
   },
   "initial": {
         "countc.RegionID":0
   },
   "reduce": function( obj , prev ){
         if (obj.c.RegionID != null ) if ( obj.c.RegionID instanceof Array)prev.countc.RegionID += obj.c.RegionID.length;
         else prev.countc.RegionID++;
   },
   "finalize": function( prev ){
   },
   "cond": {
 "$where": "this.c.RegionID  == this. r._id "
   }
});

I have not tested this, but I hope it serves as a starting point to build your query.

Answer 3

Despite of my comment I try to give a translation (not tested):

db.region.aggregate([
{
   $lookup: // left outer join collections
     {
       from: "company",
       localField: "_id",
       foreignField: "RegionID",
       as: "c"
     }
},
{ $match: { c: { $ne: [] } } }, // remove non-matching documents (i.e. INNER JOIN)
{ $group: { _id: "$regionName", regions: { $addToSet: { "$c.RegionID" } } } }, // group and get distinct regions
{ $project: { regionName: "$_id", count: { $size: "$regions" } , _id: 0} } // some cosmetic and count
{ $sort: { regionName: 1 } }, // order result
{ $limit: 5 } // limit number or returned documents
])