**CROSS APPLY**: Invokes table-valued function/subquery per row; like INNER JOIN—excludes rows where function returns nothing. **OUTER APPLY**: Like LEFT JOIN—keeps all left rows, NULL where function returns nothing. **Use**: Correlated subqueries, JSON parsing, string...
Pro-Move: 'We use CROSS APPLY for JSON parsing in SQL Server—moved from 10 UDF calls to single APPLY; 3× faster.' Red Flag: Using APPLY for set-based operations—often a JOIN is better.
This medium-level General/Other question appears frequently in data engineering interviews at companies like Kaseya. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join, sql) will help you answer variations of this question confidently.
Break this problem into components. Identify the core trade-offs involved, then walk the interviewer through your reasoning step by step. Demonstrate awareness of edge cases and production considerations - this is what separates good answers from great ones.
CROSS APPLY: Invokes table-valued function/subquery per row; like INNER JOIN—excludes rows where function returns nothing. OUTER APPLY: Like LEFT JOIN—keeps all left rows, NULL where function returns nothing. Use: Correlated subqueries, JSON parsing, string splitting. PostgreSQL: LATERAL JOIN (CROSS JOIN LATERAL, LEFT JOIN LATERAL). Performance: Can be expensive—row-by-row. Prefer set-based when possible. Best practice: Use when row-by-row needed; avoid for large tables without indexing.
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Analyze My Answer — FreeAccording to DataEngPrep.tech, this is one of the most frequently asked General/Other interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.