How does the join step help you catch data problems?

The join step shows a Venn diagram with counts — how many values matched and how many are unique to each side — before you commit to the join.

Clicking the unmatched portion shows exactly which values failed, in the profile pane. Nine times out of ten they are trailing whitespace, case differences or a formatting variation, and the fix is a clean step on the key beforehand.

That is a genuine improvement on SQL, where an inner join silently discards non-matches and you only notice when a total looks low.

The other thing to watch is the resulting row count. If it exceeds the left input, the right side has duplicate keys and rows have fanned out — fix it with an aggregate step on that branch.