# Calculations vs Aggregations

A calculation works within one row and returns a value for every row; an aggregation works across many rows and returns one value for all of them.

Apprentice · T-SQL · SQL · Updated November 3, 2026

## After this lesson you can

- Tell a calculation (one value per row) from an aggregation (one value for many rows)

## Key takeaways

- A calculation such as `UnitPrice * Quantity` uses the values in one row and gives an answer for every row.
- An aggregation such as `AVG(Quantity)` works across many rows and returns one row, however many rows went in.
- `COUNT`, `SUM`, `AVG`, `MIN` and `MAX` are aggregate functions.
- The two work together: `SUM(UnitPrice * Quantity)` works out each line's total, then adds them into one grand total.
- Before you write the query, ask whether you want an answer for every row or one answer about all of them.

There are two kinds of arithmetic in a query, and they give very different results.

## A calculation: one answer per row

A **calculation** works with the values in **one row**. It can't see any other row. This query works out each order line's total, so every row in `OrderDetails` gets its own answer:

```sql
SELECT TOP (4) OrderId, ProductId, UnitPrice * Quantity AS LineTotal
FROM OrderDetails
ORDER BY OrderId, ProductId;
```

| OrderId | ProductId | LineTotal |
|---|---|---|
| 10248 | 11 | 168.00 |
| 10248 | 42 | 98.00 |
| 10248 | 72 | 174.00 |
| 10249 | 14 | 167.40 |

Without the `TOP`, it returns all 2,155 rows of `OrderDetails`, each with its own line total.

## An aggregation: one answer for many rows

An **aggregation** works **across rows**: it takes a column from many rows and boils it down to one value. This query asks for the average quantity on an order line, across every line of every order:

```sql
SELECT AVG(Quantity) AS AverageQuantity
FROM OrderDetails;
```

| AverageQuantity |
|---|
| 23 |

**One row**, however many rows went in. `AVG` is an **aggregate function**, and so are `COUNT`, `SUM`, `MIN` and `MAX`, the subject of the next lesson.

## Side by side

| | Calculation | Aggregation |
|---|---|---|
| Works with | Values in the same row | One column, across many rows |
| Rows in the result | One per row | One for all of them (or one per group, with `GROUP BY`) |
| Example | `UnitPrice * Quantity` | `AVG(Quantity)` |
| Answers | "What's this line's total?" | "What's the average line quantity?" |

They can work together: `SUM(UnitPrice * Quantity)` calculates each line's total, then aggregates them into one grand total.

## Check yourself

1 question to try on the lesson page.

## So what

Ask yourself whether you want an answer for every row, or one answer about all of them. The first is a calculation; the second is an aggregation, and it's how databases turn thousands of rows into a single number a manager can use.

## Key terms

- [Aggregate Function](https://www.ask-codey.com/glossary/aggregate-function)

## Cheat sheets

- [Calculations vs Aggregations](https://www.ask-codey.com/cheat-sheets/aggregate-functions-in-t-sql-calculations-vs-aggregations-cheat-sheet)

Source: https://www.ask-codey.com/lessons/aggregate-functions-in-t-sql-calculations-vs-aggregations