What is MongoDB aggregation pipeline?

The aggregation pipeline processes documents through a sequence of stages, similar to SQL's GROUP BY + JOINs.

**Common stages:**
- `` — filter documents (like WHERE)
- `` — group and aggregate (like GROUP BY)
- `` — sort results
- `` — join with another collection (like JOIN)
- `` — select/rename fields (like SELECT)
- `` / `` — pagination

**Example — total sales by category:**
```javascript
db.orders.aggregate([
{ : { status: 'completed' } }, // filter
{ : {
_id: '',
totalRevenue: { : '' },
count: { : 1 }
}},
{ : { totalRevenue: -1 } }, // highest first
{ : 5 } // top 5
]);
```

Aggregation is one of MongoDB's most powerful features — learn it well for senior roles.