How well do you know SQL?

Senior-level SQL knowledge implies a deep understanding of:

• Basic operations: SELECT, INSERT, UPDATE, DELETE
• Complex queries with JOINs, subqueries, aggregate functions
• Query optimization: indexes, execution plans, performance analysis
• Transactions and isolation levels
• Procedures and triggers in DBMS
• Normalization and denormalization of data
• Working with various DBMSs and their features

For example, the ability to write an optimal query with multiple JOINs and aggregates:

SELECT c.customer_id, c.name, SUM(o.amount) AS total_spent
FROM customers c
JOIN orders o ON c.customer_id = o.customer_id
WHERE o.order_date >= '2023-01-01'
GROUP BY c.customer_id, c.name
HAVING SUM(o.amount) > 1000
ORDER BY total_spent DESC;

It is also important to analyze execution plans and optimize queries to improve performance.