Fan-out join, in short

Added a note to Fan-out join today, which is a good excuse to say the short version here.

A join to a table with more than one row per key multiplies every measure on the other side. Nothing errors, and the number is plausible.

What makes it a pattern rather than a tip is that the wrong version is the one you write naturally. It reads correctly, it runs, and it returns something. The failure is in the result, not in the execution — which means the only defence is recognising the shape before you are in it.

Two datasets on this site have the shape built in: retail-orders and world-indicators. Both are small enough to run the broken version, see the number, then run the corrected one and see it change.

It lives under shaping because the fix is in how the rows are arranged, not in the arithmetic. Almost every "the number is wrong" report that turns out to be real lands here.

The long-form treatment is in the course (joins-that-behave, rows-grain-shape); the pattern page is the version to read at 4pm with a query open.

The pattern page has the version that holds and the version that looks right and is not, side by side. Reading them together is the point; either one alone is just code.