Mastering SQL for Interviews: Essential Questions and Advanced Techniques

Here are the key SQL resources from the article to help you prepare for technical interviews and master advanced SQL concepts:

1. Beauty of SQL RANK Function | SQL Interview Question and Answers | Covid Cases
   https://lnkd.in/gycjXSBh

2. Deadly Combination of Group By and Having Clause in SQL | SQL Interview Questions and Answers
   https://lnkd.in/g_PnDzsk

3. Double Self Join in SQL | Amazon Interview Question | Excel Explanation Included | Data Analytics
   https://lnkd.in/gF8vZ_Y8

4. Recursive CTE | Leetcode Hard SQL Problem 5 | Complex SQL 12
   https://lnkd.in/g4C8adjr

5. SQL Convert Rows to Columns and Columns to Rows without using Pivot Functions
   https://lnkd.in/g4yRJG-V

6. SQL Cross Join | Use Cases | Master Data | Performance Data
   https://lnkd.in/gcCdcfj2

7. Slowly Changing Dimensions In Data Warehousing with iPhone 11 Example | SCD 1/2/3
   https://lnkd.in/ghs6P652

8. Difference Between count(), count(0), count(-1), count(col), count(‘youtube’) | SQL Interview Question
   https://lnkd.in/gf4m4zWn

9. RANK, DENSE_RANK, ROW_NUMBER SQL Analytical Functions Simplified

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1. SQL Full Outer Join Using UNION For MySQL
   https://lnkd.in/g-a7s46M

2. Aggregation and Window Functions Together in a Single SQL | Advanced SQL
   https://lnkd.in/gcAgUN25

3. first_value and last_value SQL Window Functions MASTER CLASS | Advanced SQL
   https://lnkd.in/gZBX8AMW

4. Rollup, Cube and Grouping Sets in SQL | Advanced SQL Tutorial | Beyond Group By
   https://lnkd.in/gHt74FP4

5. SCD Type 1 and Type 2 using SQL | Implementation of Slowly Changing Dimensions
   https://lnkd.in/gWaGPad3

You Should Know: Practical SQL Commands & Techniques

1. RANK, DENSE_RANK, and ROW_NUMBER

SELECT 
employee_id, 
salary, 
RANK() OVER (ORDER BY salary DESC) AS rank_salary, 
DENSE_RANK() OVER (ORDER BY salary DESC) AS dense_rank_salary, 
ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_num 
FROM employees; 

2. Recursive CTE for Hierarchical Data

WITH RECURSIVE employee_hierarchy AS ( 
SELECT id, name, manager_id, 1 AS level 
FROM employees 
WHERE manager_id IS NULL 
UNION ALL 
SELECT e.id, e.name, e.manager_id, eh.level + 1 
FROM employees e 
JOIN employee_hierarchy eh ON e.manager_id = eh.id 
) 
SELECT  FROM employee_hierarchy; 

3. Pivoting Data Without PIVOT (Rows to Columns)

SELECT 
product_id, 
MAX(CASE WHEN attribute = 'color' THEN value END) AS color, 
MAX(CASE WHEN attribute = 'size' THEN value END) AS size 
FROM product_attributes 
GROUP BY product_id; 

4. Using GROUP BY with HAVING

SELECT department_id, AVG(salary) AS avg_salary 
FROM employees 
GROUP BY department_id 
HAVING AVG(salary) > 50000; 

5. Self-Join for Finding Duplicates

SELECT a.id, a.email 
FROM users a 
JOIN users b ON a.email = b.email AND a.id <> b.id; 

6. Full Outer Join Alternative in MySQL

SELECT  FROM table1 
LEFT JOIN table2 ON table1.id = table2.id 
UNION 
SELECT  FROM table1 
RIGHT JOIN table2 ON table1.id = table2.id 
WHERE table1.id IS NULL; 

7. Window Functions with Aggregations

SELECT 
department_id, 
employee_id, 
salary, 
AVG(salary) OVER (PARTITION BY department_id) AS dept_avg_salary 
FROM employees; 

What Undercode Say

SQL remains a fundamental skill for data engineers, analysts, and developers. Mastering window functions, joins, and advanced aggregations can significantly improve query efficiency. Practice recursive CTEs for hierarchical data, and understand the nuances of `RANK()` vs DENSE_RANK(). For database optimization, always analyze query execution plans and index critical columns.

Linux/IT Commands Related to SQL & Databases:

 Check running PostgreSQL processes 
ps aux | grep postgres

Backup a MySQL database 
mysqldump -u [bash] -p [bash] > backup.sql

Monitor PostgreSQL queries in real-time 
sudo tail -f /var/log/postgresql/postgresql-[bash]-main.log

Check disk space for databases 
df -h /var/lib/mysql

Kill a long-running SQL query in PostgreSQL 
SELECT pg_cancel_backend(pid) FROM pg_stat_activity WHERE query LIKE '%long_query%';

Optimize SQLite database 
sqlite3 database.db "VACUUM; ANALYZE;"

Check SQL Server performance (Linux) 
sudo apt install mssql-tools 
sqlcmd -S localhost -U SA -Q "SELECT  FROM sys.dm_exec_requests" 

Expected Output:

A well-prepared SQL expert should be able to:

• Write complex joins and subqueries efficiently.
• Optimize queries using indexing and execution plans.
• Implement SCD (Slowly Changing Dimensions) in data warehousing.
• Use window functions for advanced analytics.
• Troubleshoot performance bottlenecks in databases.

Keep practicing! 🚀

References:

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