thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Data & Analytics · Advanced

Fan Trap

A modeling or join error in which joining tables at different grains duplicates rows and inflates aggregated totals.

Definition

If Orders (with OrderTotal) is joined to OrderLines, OrderTotal repeats for every line and SUM overstates revenue. The fix is to aggregate each table to a common grain before joining, or to model at the line grain.

Exam tip

Inflated totals after a join are almost always a grain mismatch (fan-out).

Related terms