Context

Analysts often use Excel VLOOKUP to pull columns from one dataset into another using a shared key. In SQL, the equivalent operation is typically done with a join, and interviewers want to know both the syntax and the reasoning.

Core Question

Explain how you would merge two datasets that share a common unique identifier using SQL instead of VLOOKUP.

Address these points:

1. Which SQL join most closely matches a standard VLOOKUP use case
2. How the shared unique identifier is used in the join condition
3. What happens when a matching row does not exist in the lookup table
4. When you would use INNER JOIN instead of LEFT JOIN
5. Common mistakes such as duplicate keys or joining on non-unique columns

Scope Guidance

The interviewer expects a practical explanation of how VLOOKUP maps to SQL joins, not a deep discussion of database internals. A strong answer should compare VLOOKUP behavior with LEFT JOIN and INNER JOIN, mention null handling, and include a simple PostgreSQL example.