Data engineering·Modelling for analysis
you joined two fact tables at different grains and every revenue figure quietly doubled.
Fan trap
Also calledfan-out join, join explosion, chasm trap
A join that multiplies rows because one side has several matches per key, inflating any measure summed afterwards. It is grain error made visible, and it is the single most common way a correct-looking SQL query returns a wrong number. The safe patterns are to aggregate one side to the join grain first, or to keep the two measures in separate queries and union them, rather than trusting a distinct.