SQL Tutorials

 

SQL Tutorials, Scripts & Best Practices

This page contains practical SQL content useful for interviews, real-time projects, ETL development, and query tuning. I have included scripts, examples, and explanations that help beginners and experienced developers.

---

🔹 1. SQL Basics

• SELECT, INSERT, UPDATE, DELETE

• WHERE, GROUP BY, HAVING

• ORDER BY, DISTINCT

• Joins (Inner, Left, Right, Full, Cross)

Example:

SELECT emp_id, emp_name, salary 
FROM employees 
WHERE salary > 50000;

---

🔹 2. Advanced SQL

• Window Functions (ROW_NUMBER, RANK, LEAD/LAG)

• Subqueries & CTEs

• Pivoting & Unpivoting

• Merge statement

• Analytic functions for ETL use cases

Example:

SELECT emp_id,
       salary,
       ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn
FROM employees;

---

🔹 3. SQL Performance Tuning

• Understanding Indexes

• How to read Execution Plans

• Remove unnecessary DISTINCT

• Use EXISTS instead of IN for large datasets

• Avoid functions on indexed columns

• Correct join order

Example (find high-cost operations):

Check for:
• Table scans  
• Key lookups  
• Sorts, Hash joins  
• Missing index suggestions  

---

🔹 4. SQL Interview Questions

• Difference between ROW_NUMBER, RANK, DENSE_RANK

• What is a clustered vs non-clustered index

• How to identify a slow query

• What is normalization

• What is a correlated subquery

---

🔹 5. SQL Practice Problems

• Find the 2nd highest salary

• Get duplicate records

• Remove duplicates

• Find employees who joined in last 90 days

• Identify customers without orders