Mastering SQL: Your Guide to Acing Common Interview Questions

Are you gearing up for an SQL interview? With the growing demand for data professionals, mastering SQL is essential for anyone looking to excel in the tech industry. In this post, we’ll cover some of the most commonly asked SQL interview questions and provide practical example queries to help you prepare. Plus, we’ll introduce a valuable resource that can elevate your SQL skills and boost your confidence in interviews!

1. Finding Duplicate Records 🎭

Question: How can you identify duplicate records in a table?

Explanation: Identifying duplicate records is vital for ensuring data integrity. You can use the GROUP BY clause combined with the HAVING clause to find records that occur more than once based on specified columns.

Example Query:

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SELECT first_name, last_name, COUNT(*) AS duplicate_count FROM employees GROUP BY first_name, last_name HAVING COUNT(*) > 1;

In this query, we are checking for duplicate first and last names in the employees table. The COUNT(*) function counts the number of occurrences, and the HAVING clause filters the results to show only those with more than one occurrence.

2. Finding the Second Highest Salary 📊

Question: How can you find the second highest salary from a salary table?

Explanation: There are various methods to find the second highest salary. One common approach is to use a subquery to first get the highest salary and then filter the results.

Example Query:

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SELECT MAX(salary) AS second_highest_salary FROM employees WHERE salary < (SELECT MAX(salary) FROM employees);

In this example, the inner query retrieves the maximum salary from the employees table. The outer query then selects the maximum salary that is less than that value, effectively giving you the second highest salary.

Alternatively, you can use the DISTINCT keyword to handle duplicates:

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SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 1;

This query orders unique salaries in descending order and uses LIMIT and OFFSET to skip the first result and return the second highest salary.

3. Understanding Joins 🔗

Question: What are the different types of joins, and can you provide an example of each?

Explanation: Joins are essential for combining rows from two or more tables based on related columns. Here are the most common types of joins:

INNER JOIN

Example Query:

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SELECT e.first_name, e.last_name, d.department_name FROM employees e INNER JOIN departments d ON e.department_id = d.id;

This query retrieves the first and last names of employees along with their department names, returning only rows where there is a match in both tables.

LEFT JOIN (or LEFT OUTER JOIN)

Example Query:

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SELECT e.first_name, e.last_name, d.department_name FROM employees e LEFT JOIN departments d ON e.department_id = d.id;

This query returns all employees and their corresponding department names. If an employee does not belong to a department, the department name will return as NULL.

RIGHT JOIN (or RIGHT OUTER JOIN)

Example Query:

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SELECT e.first_name, e.last_name, d.department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.id;

This query returns all departments, including those that do not have employees. If a department has no employees, the employee columns will return as NULL.

FULL JOIN (or FULL OUTER JOIN)

Example Query:

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SELECT e.first_name, e.last_name, d.department_name FROM employees e FULL OUTER JOIN departments d ON e.department_id = d.id;

This query returns all records from both tables, with NULL in places where there is no match.

4. Finding Duplicate Records with Additional Conditions 🔍

Question: How can you find duplicate records while considering additional conditions?

Example Query:

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SELECT first_name, last_name, COUNT(*) AS duplicate_count FROM employees WHERE active = 1 GROUP BY first_name, last_name HAVING COUNT(*) > 1;

In this case, we’re checking for duplicates among active employees only.

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