# What AQL is for

> AQL exists to define and query metrics: reusable aggregations that adapt to wherever you drop them. Everything else in Foundations is the plumbing that makes that possible.

Before we touch pipes, filters, or grouping, it's worth seeing the destination. AQL exists to do one thing well: **define metrics once, then query them anywhere.**

## A metric in 10 lines

A **metric** is a named, reusable aggregation. Here's one:

```aml
metric total_revenue {
  label: 'Total Revenue'
  type: 'number'
  definition: @aql sum(order_items.quantity * products.price) ;;
}
```

That's it. No `GROUP BY`, no `JOIN`, no per-report scaffolding. Just the calculation. The metric travels with its definition.

## Same metric, three reports

Drop `total_revenue` into three different reports and it adapts to each:

```aml
// Report 1: no grouping → grand total
explore { measures { total_revenue } }
// → $4,283,512

// Report 2: grouped by country → revenue per country
explore { dimensions { countries.name } measures { total_revenue } }
// → US: $2.1M, UK: $890K, ...

// Report 3: grouped by month, filtered to 2024 → monthly trend
explore {
  dimensions { orders.created_at | month() }
  measures { total_revenue }
  filters { orders.created_at matches @2024 }
}
// → Jan 2024: $312K, Feb 2024: $358K, ...
```

Same definition. Three different numbers, three different shapes. That's the part SQL can't do cleanly. In SQL you'd write three separate queries with copy-pasted aggregation logic, and any change to "what revenue means" would mean editing all three.

## What the rest of Foundations teaches

To write metrics like `total_revenue`, you need to know the primitives that go inside the definition:

- **[The pipe operator](/as-code/aql/learn/pipe)**: how steps chain together.
- **[Filtering](/as-code/aql/learn/filtering)**: narrowing rows before or inside an aggregation.
- **[Grouping and aggregation](/as-code/aql/learn/grouping)**: turning many rows into a single number.

Once those click, the [Metrics](/as-code/aql/#learning-roadmap) group covers how metrics interact with related models, how to name and reuse them, and how the surrounding report context shapes what they return.

## Next

→ [The pipe operator](/as-code/aql/learn/pipe): the single biggest reason AQL reads differently from SQL.