SQL JOIN questions that actually come up in Data Engineer interviews

After going through several DE interviews, these were the JOIN

scenarios that kept coming up:

1. "What's the difference between LEFT JOIN and INNER JOIN?"

They want to hear about NULL handling, not just definitions

2. "What happens when you JOIN on a non-unique key?"

   Duplicate rows! Always check cardinality first

3. "How would you join 3+ tables efficiently?"

   Mention filtering early, joining on indexed columns

4. "When would you avoid a JOIN entirely?"

   Subqueries or window functions sometimes perform better

The key in interviews isn't just knowing the syntax

it's explaining the WHY behind your JOIN choice.

What other SQL questions have you seen in DE interviews?