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As someone who has spent years working with SQL and has faced my fair share of challenging SQL interview questions, I understand the importance of being well-prepared. SQL (Structured Query Language) is a fundamental skill for anyone working with data, and mastering it can significantly enhance your career opportunities.
In this article, I will share some of the most advanced SQL interview questions and provide code snippets and explanations to help you tackle them confidently.
Question: Explain the different types of SQL joins and provide an example for each.
Answer:
SQL joins allow you to combine rows from two or more tables based on a related column between them. There are four main types of joins:
INNER JOIN: Returns only the rows that have matching values in both tables.
SELECT employees.name, departments.department_name
FROM employees INNER JOIN departments ON employees.department_id = departments.id;
LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table and the matched rows from the right table. If there is no match, NULL values are returned for the right table’s columns.
SELECT customers.customer_name, orders.order_date
FROM customers LEFT JOIN orders ON customers.customer_id = orders.customer_id;
RIGHT JOIN (or RIGHT OUTER JOIN): Similar to LEFT JOIN but returns all rows from the right table and matched rows from the left table.
SELECT employees.name, salaries.salary_amount
FROM employees RIGHT JOIN salaries ON employees.employee_id = salaries.employee_id;
FULL OUTER JOIN: Returns all rows when there is a match in either the left or right table. If there is no match, NULL values are returned for the missing side.
SELECT customers.customer_name, orders.order_date
FROM customers FULL OUTER JOIN orders
ON customers.customer_id = orders.customer_id;
Question: What is a subquery in SQL, and how can you use it to retrieve specific data?
Answer: