Convert SQL query into mongodb

Question

What is the MongoDB equivalent for the following SQL query?

SELECT
  station_id,
  group_concat('signals')
FROM signals
GROUP BY station_id

i need to group by "station id", and concat "signal", thanks, the result would be like that:

| station_id | signals |
     321       2,3,5,2

Answer 1

Please refer following aggregate query as an equivalent query of SQL into Mongodb

db.signals.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $group: {
                _id: '$station_id',
                signals: {
                    $push: '$signal'
                }
            }
        },

    ]

);