Need to convert this SQL query to MongoDB - sql

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

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
])