# Tables and rows

> AQL operates on tables, one row at a time. Understanding the 'current row' is the foundation for everything else.

AQL is a data query language. Like SQL or Excel, it operates on **tables**: collections of rows, where each row has columns. Before we touch any function, it helps to get precise about what AQL actually does as it reads your data.

<VideoPlayer src="https://cdn.holistics.io/docs/as-code/concepts/row/table.mp4" />

:::tip Try it interactively
Practice in the AQL Playground: [Metric Expressions](https://play.amql.org/learn/metric-expressions).
:::

## A tiny example

Imagine an `order_items` table:

| id | order_id | product | quantity | price |
|----|----------|---------|----------|-------|
| 1  | 1001     | Mug     | 2        | 10    |
| 2  | 1001     | Pen     | 5        | 2     |
| 3  | 1002     | Book    | 1        | 25    |

Three rows, five columns. AQL operates on tables like this one.

## The current row

Unlike Excel, you can't point at a single cell. The smallest thing AQL works on is a **row**. When AQL processes a table, it walks through the rows one at a time. The row it's currently looking at is called the **current row**.

<VideoPlayer src="https://cdn.holistics.io/docs/as-code/concepts/row/iteration2.mp4" />

Inside the current row, you refer to columns by name. So `order_items.quantity` means "the quantity column of whatever row I'm on right now". As AQL walks the table above, that expression evaluates to `2`, then `5`, then `1`.

You can combine columns too. `order_items.quantity * order_items.price` is a **row-level expression**: it runs once per row, giving `20`, then `10`, then `25`.

## Iteration

That walk-through-the-rows process is called **iteration**. Most AQL functions iterate. A few you'll meet next:

- [`select`](/reference/aql/select): walks rows, produces a new table with just the columns you ask for.
- [`filter`](/reference/aql/filter): walks rows, keeps only the ones matching a condition.
- [`sum`](/reference/aql/aggregator-functions#sum): walks rows, adds up a column into one number.

For example, the total revenue across the table:

```aml
order_items | sum(order_items.quantity * order_items.price)
// walks all 3 rows, computes 20 + 10 + 25 = 55
```

Don't worry about the `|` yet (that's the next page). The point for now: any expression you hand to one of these functions is evaluated **once per row**.

## Next

→ [The pipe operator](/as-code/aql/learn/pipe): how AQL chains operations together.