group by

SUM() Aggregation With GROUP BY

Sum total units ordered per product category using SUM() grouped by category.

Overview

SUM() adds up a numeric column across rows. Paired with GROUP BY, it answers "total X per Y" questions — such as total units sold per product category in the store.

The Query

Store (Customers/Orders/Products)

Loading database engine...

Step-by-Step Breakdown

SUM(o.quantity) totals units sold per category.
SUM(o.quantity * p.price) computes revenue per category by multiplying before summing.
GROUP BY p.category defines the buckets.

Aggregate then compute

SUM(quantity * price) multiplies inside each row, then sums — different from SUM(quantity) * SUM(price), which would be wrong.

Variations

Sum order quantities per customer city:

Store (Customers/Orders/Products)

Loading database engine...

Common Mistakes

SUM of a text column. SUM requires numbers; summing a name errors.
Mixing SUM with non-grouped columns. Every selected column must be grouped or aggregated.
Ignoring NULLs. SUM ignores NULL values, which is usually what you want, but be aware it won't treat them as zero.