SQL Fundamentals and Query Writing

Prompt SQL round: concept questions on database fundamentals followed by writing real queries.

Be ready to discuss

• Keys and constraints: primary vs unique vs foreign keys, NOT NULL/CHECK/DEFAULT, referential actions (CASCADE/SET NULL).
• Indexes: B-tree basics, when an index helps vs hurts (write overhead, low-selectivity columns), composite index column order, covering indexes.
• Joins: INNER vs LEFT/RIGHT vs FULL, join vs subquery, and NULL pitfalls in joins and comparisons.
• Aggregation: GROUP BY semantics, HAVING vs WHERE, window functions (ROW_NUMBER/RANK, running totals).
• Typical queries to write on the spot: second-highest salary, top-N per group (window + filter), duplicate detection (GROUP BY … HAVING count > 1), employees without a manager (LEFT JOIN … IS NULL), department-wise averages.