UNION in relational algebra?
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of SQL & Databases conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
In relational algebra, the UNION of two sets $A \cup B$ combines all distinct tuples from both relations into a single unified relation.
### UNION vs UNION ALL in SQL:
-- UNION removes duplicate records (performs sorting/deduplication)
SELECT city FROM customers
UNION
SELECT city FROM suppliers;
-- UNION ALL retains all rows including duplicates (much faster)
SELECT city FROM customers
UNION ALL
SELECT city FROM suppliers;
*Performance rule:* Always prefer UNION ALL unless business logic strictly requires deduplicating rows across the combined sets.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.