Top 3 Joins Interview Questions

Browse 3 real-world technical and behavioral interview questions about Joins. Review scenarios, edge cases, and architectural best practices.

Related Topics:Aggregation Fan out Sql Postgresql Time series

Adding a second LEFT JOIN doubled the revenue figure on a report. Explain what happened and write the correct query.

SQL join duplicates cause double counting when two one-to-many joins multiply rows before aggregation. Aggregate each child table to one row per parent first, then join those results back to the parent. Use this sql queries answer to show the decision, trade-off, and evidence rather than a memorised definition.

mediumcoding

Find me every customer with no order in the last thirty days, then tell me which ways of writing that get NULLs wrong.

Use NOT EXISTS for customers with no recent order because it expresses an anti-join and avoids NULL traps. NOT IN can return no rows if the subquery contains NULL, and LEFT JOIN becomes wrong if filters are placed in WHERE. Use this SQL answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects null semantics to the point an interviewer is testing.

mediumcoding

One machine's clock is ninety seconds ahead of the rest of the plant. Which of your numbers are wrong, and how would you have found out?

Every derived figure that joins on time is affected, and none of them error. Duration and attribution metrics take the skew directly, so a shift with twelve stops mis-measures availability by nearly four points, and only storing both event time and ingestion time makes the offset visible.

hardscenario