Lesson 3 of 13 · 18 min
GROUP BY: one row per group
By the end of this lesson you can write GROUP BY queries that return one summary row per category, and explain (and fix) the “must appear in the GROUP BY clause” error.
In the last two lessons an aggregate query turned the whole table into one row. Most business questions want a breakdown instead: how many orders per channel? That needs one count for web, one for app and one for marketplace.
GROUP BY channel tells PostgreSQL to sort the rows into groups (think of buckets), one group for each different value of channel. Every aggregate in the SELECT list is then computed separately for each group, and each group becomes one output row.
This is the single most important mental model in reporting: after GROUP BY, you are no longer working with rows, you are working with groups.
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