How would you handle nulls in a SQL join? Provide examples using COALESCE.

Nulls in SQL joins behave uniquely: NULL = NULL evaluates to NULL (not TRUE), so join keys with nulls typically don't match. Use COALESCE to normalize join keys. Example: SELECT FROM orders o JOIN customers c ON COALESCE(o.customer_id, -1) = COALESCE(c.id, -1). For left joins where right-side columns may be null, use COALESCE in SELECT: COALESCE(c.name, 'Unknown Customer'). To include null-matching rows explicitly, use: ON (o.key = c.key OR (o.key IS NULL AND c.key IS NULL)). Best practice: decide if null keys should match (rare) or be excluded. Use COALESCE when consolidating from multiple sources with different null conventions. Example: SELECT o., COALESCE(c.country, 'N/A') FROM orders o LEFT JOIN customers c ON o.customer_id = c.id; Why it matters: Design choices compound at scale—wrong approach can cause 100× overhead. Scalability trade-offs: Profile before optimizing; validate on sample then full. Cost implications: Suboptimal choices multiply at billion-row scale.