Why is $match not used in the Mongo Aggregation query? - mongodb

Question

As described in the mongo documentation:
https://docs.mongodb.com/manual/reference/sql-aggregation-comparison/
There is a query for the following SQL query:
SELECT cust_id,
SUM(li.qty) as qty
FROM orders o,
order_lineitem li
WHERE li.order_id = o.id
GROUP BY cust_id
And the equivalent mongo aggregation query is as follows:
db.orders.aggregate( [
{ $unwind: "$items" },
{
$group: {
_id: "$cust_id",
qty: { $sum: "$items.qty" }
}
}
] )
However, the query is workinf fine as expected. My question, why is there no $match clause for the corresponding WHERE clause in SQL? And how is $unwind compensating the $match clause?

Answer 1

The comment by #Veeram is correct. The where clause in the SQL is unnecessary because the items list is embedded in the orders collection, where in a relational database you would have both an orders table and an orders_lineitem table (names taken from the description at https://docs.mongodb.com/manual/reference/sql-aggregation-comparison/)
Per the example data, you start with documents like this:
{
cust_id: "abc123",
ord_date: ISODate("2012-11-02T17:04:11.102Z"),
status: 'A',
price: 50,
items: [ { sku: "xxx", qty: 25, price: 1 },
{ sku: "yyy", qty: 25, price: 1 } ]
}
When you $unwind, the items are unwound but the rest of the data is projected. If you run a query like
db.orders.aggregate([ {"$unwind": "$items"} ])
you get the output
{
cust_id: "abc123",
ord_date: ISODate("2012-11-02T17:04:11.102Z"),
status: 'A',
price: 50,
items: { sku: "xxx", qty: 25, price: 1 }
},
{
cust_id: "abc123",
ord_date: ISODate("2012-11-02T17:04:11.102Z"),
status: 'A',
price: 50,
items: { sku: "yyy", qty: 25, price: 1 }
}
That has flattened the items array, allowing the $group to add the items.qty field:
db.orders.aggregate([
{"$unwind": "$items"},
{"$group": {
"_id": "$cust_id",
"qty": {"$sum": "$items.qty"}
}
}])
With the output:
{ "_id": "abc123",
"qty": 50
}