Data Analytics

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SQL Interview Questions with detailed answers: 1️⃣1️⃣ How does a self-join work? Give an example. A self-join is when a table joins with itself. It is useful for comparing rows within the same table, such as finding employees and their managers. Example: Find Employee-Manager Relationships

SELECT e1.employee_id AS Employee, e1.name AS Employee_Name, e2.employee_id AS Manager, e2.name AS Manager_Name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.employee_id; 

Explanation: e1 represents employees. e2 represents managers. The join condition e1.manager_id = e2.employee_id matches employees to their managers. Top 20 SQL Interview Questions Like this post if you want me to continue this SQL Interview Series♥️ Share with credits: https://t.me/sqlspecialist Hope it helps :)

SQL Interview Questions with detailed answers: 1️⃣1️⃣ How does a self-join work? Give an example. A self-join is when a table joins with itself. It is useful for comparing rows within the same table, such as finding employees and their managers. Example: Find Employee-Manager Relationships

SELECT e1.employee_id AS Employee, e1.name AS Employee_Name, e2.employee_id AS Manager, e2.name AS Manager_Name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.employee_id; 

Explanation: e1 represents employees. e2 represents managers. The join condition e1.manager_id = e2.employee_id matches employees to their managers. Top 20 SQL Interview Questions Like this post if you want me to continue this SQL Interview Series♥️ Share with credits: https://t.me/sqlspecialist Hope it helps :)

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Changed the channel name from "Data Analysts" to "Data Analytics" as moving further I've decided to also teach Data Science, AI, and the latest industry trends to help you stay ahead! If you support this change, drop a like ❤️

If you want to Excel at using the most used database language in the world, learn these powerful SQL features: • Wildcards (%, _) – Flexible pattern matching • Window Functions – ROW_NUMBER(), RANK(), DENSE_RANK(), LEAD(), LAG() • Common Table Expressions (CTEs) – WITH for better readability • Recursive Queries – Handle hierarchical data • STRING Functions – LEFT(), RIGHT(), LEN(), TRIM(), UPPER(), LOWER() • Date Functions – DATEDIFF(), DATEADD(), FORMAT() • Pivot & Unpivot – Transform row data into columns • Aggregate Functions – SUM(), AVG(), COUNT(), MIN(), MAX() • Joins & Self Joins – Master INNER, LEFT, RIGHT, FULL, SELF JOIN • Indexing – Speed up queries with CREATE INDEX Like it if you need a complete tutorial on all these topics! 👍❤️

SQL Interview Questions with detailed answers: 🔟 How do you calculate a running total in SQL? A running total (also known as a cumulative sum) is the sum of values up to the current row. You can calculate it using window functions like SUM() OVER(). Using SUM() with OVER() (Best Approach)

SELECT employee_id, salary, SUM(salary) OVER (ORDER BY employee_id) AS running_total FROM employees;

Explanation: SUM(salary) OVER (ORDER BY employee_id) calculates a cumulative sum. The ORDER BY employee_id ensures the total is calculated sequentially. Running Total Partitioned by a Category To calculate the running total within groups (e.g., per department): 👇

SELECT department_id, employee_id, salary, SUM(salary) OVER (PARTITION BY department_id ORDER BY employee_id) AS running_total FROM employees;

Top 20 SQL Interview Questions Like this post if you want me to continue this SQL Interview Series♥️ Share with credits: https://t.me/sqlspecialist Hope it helps :)

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SQL Interview Questions with detailed answers: 9️⃣ What is the difference between UNION and UNION ALL? Both UNION and UNION ALL are used to combine the results of two or more SELECT queries, but they handle duplicate records differently. 1️⃣ UNION (Removes Duplicates) Combines result sets and removes duplicate rows automatically. It performs an implicit DISTINCT operation, which may affect performance.

SELECT employee_id, department_id FROM employees UNION SELECT employee_id, department_id FROM managers; 

2️⃣ UNION ALL (Keeps Duplicates) Combines result sets without removing duplicates. Faster than UNION because it doesn’t perform duplicate elimination.

SELECT employee_id, department_id FROM employees UNION ALL SELECT employee_id, department_id FROM managers; 

Key Differences: UNION removes duplicates, which may cause performance overhead. UNION ALL keeps all records, making it more efficient. Top 20 SQL Interview Questions Like this post if you want me to continue this SQL Interview Series♥️ Share with credits: https://t.me/sqlspecialist Hope it helps :)

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SQL Interview Questions with detailed answers: 8️⃣ How do you identify missing values in a dataset using SQL? In SQL, missing values are usually represented as NULL. You can detect them using the IS NULL condition. Basic Query to Find NULL Values in a Column

SELECT * FROM employees WHERE salary IS NULL; 

This retrieves all employees where the salary is missing. Find Missing Values in Multiple Columns

SELECT * FROM employees WHERE salary IS NULL OR department_id IS NULL; 

This checks for NULL values in both the salary and department_id columns. Count Missing Values in Each Column

SELECT COUNT(*) AS total_rows, COUNT(salary) AS non_null_salaries, COUNT(department_id) AS non_null_departments FROM employees; 

Since COUNT(column_name) ignores NULL values, subtracting it from COUNT(*) gives the number of missing values. Top 20 SQL Interview Questions Like this post if you want me to continue this SQL Interview Series♥️ Share with credits: https://t.me/sqlspecialist Hope it helps :)

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