MongoDB Aggregation Match
Example
How to write a query to get all departments where average age of employees making less than or $70000 is greather than or equal to 35?
In order to that we need to write a query to match employees that have a salary that is less than or equal to $70000. Then add the aggregate stage to group the employees by the department. Then add an accumulator with a field named e.g. average_age to find the average age per department using the $avg accumulator and below the existing $match and $group aggregates add another $match aggregate so that we're only retrieving results with an average_age that is greather than or equal to 35.
db.employees.aggregate([
{"$match": {"salary": {"$lte": 70000}}},
{"$group": {"_id": "$dept",
"average_age": {"$avg": "$age"}
}
},
{"$match": {"average_age": {"$gte": 35}}}
])
The result is:
{
"_id": "IT",
"average_age": 31
}
{
"_id": "Customer Service",
"average_age": 34.5
}
{
"_id": "Finance",
"average_age": 32.5
}
