Find all employees whose salary is strictly greater than their manager's salary. Return employee names.
This self-join problem is a classic in Microsoft interviews, testing your understanding of hierarchical data and self-referential relationships. In organizational databases, employees and managers are stored in the same table, requiring you to join the table to itself. This pattern appears frequently in tree-structured data like org charts, file systems, and category hierarchies. The challenge is conceptualizing the same table playing two different roles in the query.
Core concepts: self-join (joining a table to itself), table aliases for disambiguation (e for employee, m for manager), foreign key relationships within the same table (manager_id references id), WHERE clause for comparison across joined rows, and handling NULL values (top-level managers have no manager_id). Understanding when and how to use self-joins is essential for hierarchical queries.
Microsoft's HR and analytics systems use similar queries to: generate organizational charts and reporting structures, identify compensation anomalies for HR review, calculate span of control metrics (how many direct reports per manager), detect circular reporting relationships in data quality checks, power manager dashboards showing team composition, and analyze promotion patterns across the organization hierarchy.
When tackling this Microsoft problem, the key is to understand the grain of the result. Are you returning one row per user, or one row per category? Always start by identifying your unique join keys and consider if filtered aggregations (CASE WHEN) are more efficient than multiple subqueries.
Be careful with NULL values in your JOIN conditions or aggregate functions. In interview scenarios, datasets often include edge cases like zero-count categories or duplicate entries that can throw off a simple COUNT(*) if not handled with DISTINCT.
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