Why should you care?

To sum revenue by SKU you need one row per item, not one row per order. $unwind makes that possible.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.orders.insertOne({
  orderId: "O1",
  items: [
    { sku: "A", price: 100, qty: 2 },
    { sku: "B", price: 50, qty: 1 }
  ]
})
db.orders.aggregate([
  { $unwind: "$items" },
  { $project: {
      orderId: 1,
      sku: "$items.sku",
      lineTotal: { $multiply: ["$items.price", "$items.qty"] }
  }}
])

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• One order becomes two documents after $unwind.
• Each has one items element.
• lineTotal multiplies price and qty per line.
• Perfect input for a later $group by sku.

Practice next

1. Run the unwind pipeline.
2. Add $group by sku summing lineTotal.
3. Try preserveNullAndEmptyArrays option for empty items.
4. Unwind tags on posts and $group to count tag popularity.
5. Chain $sort by lineTotal.

Remember

$unwind flattens arrays to rows. Essential before per-element math. Can explode document counts.

SKU contribution report

Finance unwinds order items to see which SKUs drove festival revenue.

Outcome: Clear ranking of top-selling SKUs.