Practice SQL Online

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section28JDBCusingSQLite/Solve Delete 7.txt renamed to Section28JDBCusingSQLite/PracticeSQL/src/General MySQL optimization tips.txt

@@ -1,46 +1,47 @@
-I can give general MySQL optimization tips, but for a specific optimization, I need to see your query. Here are some common ways to optimize SQL queries in MySQL:
+General MySQL optimization tips.
+
+Here are some common ways to optimize SQL queries in MySQL:
 
 1. Use Indexing Properly
 Ensure that frequently used columns in WHERE, JOIN, and ORDER BY clauses have indexes.
 Use composite indexes if filtering by multiple columns.
-2. **Avoid SELECT ***
+
+2. Avoid SELECT
 Select only the necessary columns to reduce I/O load.
+
 Example:
-sql
-Copy
-Edit
+
 SELECT id, name FROM users WHERE status = 'active';
+
 3. Use EXPLAIN for Query Analysis
+
 Run EXPLAIN before your query to check execution plans:
-sql
-Copy
-Edit
+
 EXPLAIN SELECT * FROM orders WHERE customer_id = 123;
 Look for "Using filesort" or "Using temporary", which indicate potential optimizations.
-4. Optimize JOINS
+
+4. Optimize JOINS:
+
 Ensure joined columns have indexes.
 Use INNER JOIN instead of LEFT JOIN when possible.
 Avoid joining large tables unnecessarily.
+
 5. Use LIMIT for Large Datasets
 Instead of fetching all rows, paginate results:
-sql
-Copy
-Edit
 SELECT * FROM products ORDER BY price DESC LIMIT 100;
+
 6. Optimize Subqueries & Use Joins Instead
 Convert correlated subqueries into JOINs when possible.
+
 Example (bad):
-sql
-Copy
-Edit
 SELECT name FROM customers WHERE id IN (SELECT customer_id FROM orders);
 Optimized version:
-sql
-Copy
-Edit
+
 SELECT DISTINCT c.name FROM customers c JOIN orders o ON c.id = o.customer_id;
+
 7. Partitioning Large Tables
 For massive datasets, consider table partitioning to improve query speed.
+
 8. Use Proper Data Types
 Use INT for numbers instead of VARCHAR.
-Use DATETIME instead of VARCHAR for date fields.
+Use DATETIME instead of VARCHAR for date fields.

Section28JDBCusingSQLite/most useful and important SQL concept.txt renamed to Section28JDBCusingSQLite/PracticeSQL/src/Important SQL Concepts.txt

@@ -1,27 +1,25 @@
-most useful and important SQL concepts you should know for efficient querying:
+the most useful and important SQL concepts you should know for efficient querying:
 
 ---
 
 SELECT Clause → Fetch Data  
    Retrieves data from a table.  
 
    SELECT column1, column2 FROM table_name;
-   Example: 
-  
+   Example:
    SELECT name, roll FROM students;
 
 ---
 
 FROM Clause → Specify Table:
-   Indicates the table from which to fetch data.  
-  
+   Indicates the table from which to fetch data.
    SELECT * FROM students;
 
 ---
 
 WHERE Clause → Filter Data  
    Retrieves rows based on conditions.  
-   
+
    SELECT * FROM students WHERE deptno = 10;
 
    Operators: 
@@ -47,57 +45,53 @@ SELECT name, roll FROM students ORDER BY name ASC;
 ---
 
 LIMIT Clause → Restrict Number of Rows  
-   Fetches a specific number of rows.  
+
+Fetches a specific number of rows.
 
-   SELECT * FROM students LIMIT 5;
+SELECT * FROM students LIMIT 5;
 
 ---
 
-JOINs (Combining Tables)*
-   INNER JOIN (common values in both tables)  
-   
-   SELECT students.name, dept.deptname 
-   FROM students 
-   
-  INNER JOIN dept ON students.deptno = dept.deptno;
+JOINs (Combining Tables)
+INNER JOIN (common values in both tables)
+
+SELECT students.name, dept.deptname
+FROM students
+INNER JOIN dept ON students.deptno = dept.deptno;
 
-   -  LEFT JOIN  (all rows from the left table, matching from right)  
-   -  RIGHT JOIN  (all rows from the right table, matching from left)  
+-  LEFT JOIN  (all rows from the left table, matching from right)
+-  RIGHT JOIN  (all rows from the right table, matching from left)
 
 ---
 
 GROUP BY & Aggregate Functions → Group & Calculate  
    - COUNT(), SUM(), AVG(), MAX(), MIN()
 
      SELECT deptno, COUNT(*) FROM students GROUP BY deptno;
-  
 ---
 
-HAVING Clause* → Filter Groups  
-   Used after GROUP BY (unlike WHERE).  
+HAVING Clause → Filter Groups
+Used after GROUP BY (unlike WHERE).
 
-   SELECT deptno, COUNT(*) 
-   FROM students 
-   GROUP BY deptno 
-   HAVING COUNT(*) > 3;
+SELECT deptno, COUNT(*)
+FROM students
+GROUP BY deptno
+HAVING COUNT(*) > 3;
 
 ---
 
 INSERT, UPDATE, DELETE (Modify Data)
-   - INSERT → Add new records  
-     
+   - INSERT → Add new records
      INSERT INTO students (roll, name, deptno) VALUES (16, 'Ravi', 30);
 
-   - UPDATE → Modify existing records  
- 
+   - UPDATE → Modify existing records
      UPDATE students SET name = 'Raj' WHERE roll = 1;
 
-   - DELETE → Remove records  
-    
+   - DELETE → Remove records
      DELETE FROM students WHERE roll = 1;
 
 ---
 
-💡 Bonus: Use Indexes to speed up searches:  
-   
-   CREATE INDEX idx_deptno ON students(deptno);
+Bonus: Use Indexes to speed up searches:
+
+CREATE INDEX idx_deptno ON students(deptno);

Section28JDBCusingSQLite/PracticeSQL/src/Practice SQL Online.txt

@@ -0,0 +1,97 @@
+🚀 2️⃣ Interactive SQL Playgrounds
+You can practice SQL online without installing anything! These platforms help you run queries directly in the browser
+and explore SQL in real time.
+
+🔹 SQL Online Editors (Run Queries Directly)
+1. SQLite Online – Best for practicing SQLite
+2. DB Fiddle – Supports MySQL, PostgreSQL, and SQLite
+3. Mode SQL Tutorial – Interactive guided SQL exercises with real examples
+4. W3Schools SQL TryIt – Basic SQL practice with simple queries
+
+🔹 Full Databases for Practice
+5. LeetCode SQL – SQL problems from real-world scenarios, interview-focused
+6. HackerRank SQL – Competitive SQL coding challenges with difficulty levels
+7. Kaggle Datasets + SQL – Use SQL on real datasets inside Kaggle Notebooks
+
+Recommendation:
+Start with SQLite Online for basic query practice.
+Move to HackerRank or LeetCode for advanced, problem-solving style SQL challenges.
+Use Kaggle for working with real-world datasets.
+
+------------------------------------------------------------
+
+🚀 3️⃣ SQL Project Ideas (Real-World Use Cases)
+Here are some cool SQL-based projects you can try. These projects simulate real-world applications where SQL plays a
+critical role.
+
+✅ 1. Student Management System (CRUD App)
+Features:
+- Add, update, and delete students
+- Assign students to departments
+- Generate reports (like total students per department)
+
+Tech Stack:
+Frontend: React.js / HTML-CSS-JS
+Backend: Flask / Node.js
+Database: PostgreSQL / MySQL
+
+Key Query:
+SELECT deptno, COUNT(*) FROM students GROUP BY deptno;
+
+------------------------------------------------------------
+
+✅ 2. Online Library System
+Features:
+- Store books, authors, and borrowers
+- Track issued and returned books
+- Generate overdue book reports
+
+Key Query:
+SELECT books.title, borrowers.name
+FROM books
+JOIN loans ON books.book_id = loans.book_id
+JOIN borrowers ON loans.borrower_id = borrowers.id
+WHERE loans.return_date IS NULL;
+
+------------------------------------------------------------
+
+✅ 3. E-commerce Sales Dashboard
+Features:
+- Track customer orders and payments
+- Show top-selling products
+- Find customers who haven’t ordered in the last 30+ days
+
+Key Query:
+SELECT product_name, SUM(quantity) AS total_sold
+FROM orders
+GROUP BY product_name
+ORDER BY total_sold DESC
+LIMIT 5;
+
+------------------------------------------------------------
+
+✅ 4. Employee Attendance System
+Features:
+- Store employee check-in and check-out times
+- Calculate total working hours per employee
+- Find employees with low attendance records
+
+Key Query:
+SELECT employee_id, SUM(hours_worked) AS total_hours
+FROM attendance
+GROUP BY employee_id
+HAVING total_hours < 40;
+
+------------------------------------------------------------
+
+✅ 5. Coding Time Tracker (Like WakaTime)
+Features:
+- Log programming time per language
+- Track daily, weekly, and monthly coding stats
+- Show most-used programming languages
+
+Key Query:
+SELECT language, SUM(time_spent) AS total_time
+FROM coding_logs
+GROUP BY language
+ORDER BY total_time DESC;

Section28JDBCusingSQLite/PracticeSQL/src/SQL Practice.txt

@@ -0,0 +1,112 @@
+real-world SQL problems:
+
+Scenario: University Database
+
+You have two tables:
+
+📌 students Table
+
+| roll | name   | city       | deptno |
+|------|--------|-----------|--------|
+| 1    | Smith  | Delhi     | 10     |
+| 2    | Vijay  | Kolkata   | 10     |
+| 3    | Amit   | Mumbai    | 20     |
+| 4    | Priya  | Chennai   | 30     |
+| 5    | Rahul  | Pune      | 40     |
+
+📌 dept Table
+
+| deptno | deptname  |
+|--------|----------|
+| 10     | CSE      |
+| 20     | ECE      |
+| 30     | Mech     |
+| 40     | Civil    |
+| 50     | Chemical |
+
+---
+
+1. Find all students from Delhi
+SELECT * FROM students WHERE city = 'Delhi';
+
+2. Retrieve names of students studying in department 10
+SELECT name FROM students WHERE deptno = 10;
+
+3. Show unique department numbers (without duplicates)
+
+SELECT DISTINCT deptno FROM students;
+
+4. List students in alphabetical order
+
+SELECT name FROM students ORDER BY name ASC;
+
+5. Count the total number of students
+
+SELECT COUNT(*) FROM students;
+
+---
+
+Intermediate Level Problems:
+
+6. Find students whose names start with "A"
+
+SELECT * FROM students WHERE name LIKE 'A%';
+
+7. Get the number of students in each department
+
+SELECT deptno, COUNT(*) FROM students GROUP BY deptno;
+
+8. Find students from either 'Delhi' or 'Mumbai'
+
+SELECT * FROM students WHERE city IN ('Delhi', 'Mumbai');
+
+9. Find students NOT from Kolkata
+
+SELECT * FROM students WHERE city <> 'Kolkata';
+
+10. Find students who have a department assigned (`deptno IS NOT NULL`)
+
+SELECT * FROM students WHERE deptno IS NOT NULL;
+
+
+Advanced Level Problems
+
+11. Retrieve student names along with their department names (JOIN)
+
+SELECT students.name, dept.deptname 
+FROM students 
+INNER JOIN dept ON students.deptno = dept.deptno;
+
+12. Show departments that have at least 2 students
+
+SELECT deptno, COUNT(*) 
+FROM students 
+GROUP BY deptno 
+HAVING COUNT(*) >= 2;
+
+13. Find the **top 3 departments** with the most students
+
+SELECT deptno, COUNT(*) 
+FROM students 
+GROUP BY deptno 
+ORDER BY COUNT(*) DESC 
+LIMIT 3;
+
+14. Find students without a department (LEFT JOIN)
+
+SELECT students.name, dept.deptname 
+FROM students 
+LEFT JOIN dept ON students.deptno = dept.deptno
+WHERE dept.deptname IS NULL;
+
+15. Delete students from the 'Civil' department
+
+DELETE FROM students WHERE deptno = (SELECT deptno FROM dept WHERE deptname = 'Civil');
+
+---
+
+Bonus Challenge:
+Write a query to find the department with the most students
+Retrieve the top 5 students based on roll number, but skip the first 2
+
+---