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How do you handle NULL values in SQL? Mention functions like COALESCE and ISNULL.

SQLmedium0.5 min read

**Architectural Logic**: NULL represents unknown/missing; it propagates through expressions (NULL + 1 = NULL). Handling: IS NULL / IS NOT NULL for predicates; COALESCE(val1, val2, ...) for first non-NULL (portable); ISNULL/IFNULL for dialect-specific default; NULLIF(val1, val2)...

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Frequency
Low
Asked at 3 companies
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
AareteDunnhumbyIncedo
Interview Pro Tip

Red Flag: Joining on nullable columns without handling—'orphan' semantics. Pro-Move: 'We use COALESCE(join_key, -1) for nullable keys and document -1 as 'unknown' in the data dictionary.'

Key Concepts Tested
joinsql

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Aarete, Dunnhumby, Incedo. 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.

How to Approach This

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.

Expert Answer
102 words

Architectural Logic: NULL represents unknown/missing; it propagates through expressions (NULL + 1 = NULL). Handling: IS NULL / IS NOT NULL for predicates; COALESCE(val1, val2, ...) for first non-NULL (portable); ISNULL/IFNULL for dialect-specific default; NULLIF(val1, val2) to normalize to NULL. Why: JOIN on NULL yields no match (NULL != NULL). Aggregates ignore NULL except COUNT(*). Explicit handling prevents silent exclusions. Scalability: COALESCE chains are cheap; avoid NULL in join keys—consider sentinel values or pre-join normalization. Cost: NULL in indexes can affect pruning; sparse columns may benefit from nullable design. Best practice: Be explicit; use COALESCE for portability; document NULL semantics in contracts.

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