SQL Practice Questions

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Database Systems & Relational Data

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SQL Hard

Database Systems & Relational Data → SQL

Consider a table ‘transactions’ with a composite index on (status, created_at). You execute the following query: SELECT id, amount FROM transactions WHERE status = ‘COMPLETED’ AND created_at > ‘2023-01-01’ OR status = ‘PENDING’; Given the operator precedence in SQL, how will the database engine likely process this filter, and what is the impact on index utilization?

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SQL Hard

Database Systems & Relational Data → SQL

What is the difference between a non-repeatable read and a dirty read?

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SQL Hard

Database Systems & Relational Data → SQL

What does the SQL PIVOT operation do?

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SQL Hard

Database Systems & Relational Data → SQL

What is the difference between TRUNCATE and DROP TABLE?

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SQL Hard

Database Systems & Relational Data → SQL

What does the SQL temporal table (system-versioned table) feature provide?

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SQL Hard

Database Systems & Relational Data → SQL

What is a covering index and how does it eliminate key lookups?

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SQL Hard

Database Systems & Relational Data → SQL

What does the following demonstrate about the EXISTS vs IN performance trade-off? — Query A: WHERE id IN (SELECT customer_id FROM orders WHERE total > 1000) — Query B: WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id AND o.total > 1000)

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SQL Hard

Database Systems & Relational Data → SQL

What is the consequence of having too many autoloaded or frequently-updated rows in the same database page?

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SQL Hard

Database Systems & Relational Data → SQL

What is the purpose of the FILTER clause in SQL aggregate functions?

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SQL Hard

Database Systems & Relational Data → SQL

What does the following query return and what performance concern does it raise? SELECT * FROM orders WHERE YEAR(order_date) = 2023

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SQL Hard

Database Systems & Relational Data → SQL

What does the following query demonstrate about NULL handling in aggregates? SELECT AVG(bonus) FROM employees

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SQL Hard

Database Systems & Relational Data → SQL

What does the SQL standard LATERAL join (or CROSS APPLY in SQL Server) enable?

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SQL Hard

Database Systems & Relational Data → SQL

What is the significance of N+1 query problem in SQL and ORM contexts?

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SQL Hard

Database Systems & Relational Data → SQL

What does the following query do? SELECT id, salary, ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) AS rn FROM employees QUALIFY rn = 1

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SQL Hard

Database Systems & Relational Data → SQL

What does the GENERATED ALWAYS AS (expression) STORED column definition do?

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SQL Hard

Database Systems & Relational Data → SQL

What does the SQL:2003 TABLESAMPLE clause provide?

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SQL Hard

Database Systems & Relational Data → SQL

What is the output of: SELECT NULL = NULL in SQL?

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SQL Hard

Database Systems & Relational Data → SQL

What does the following pattern detect? SELECT a.id FROM tableA a WHERE NOT EXISTS (SELECT 1 FROM tableB b WHERE b.a_id = a.id)

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