day4_sql_dml_queries.md

📘 Day 4: SQL DML & Query Commands – INSERT, SELECT, WHERE, UPDATE, DELETE (Complete Notes)

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🗂️ Aaj Ke Topics

Command	Purpose
INSERT	Naya data add karna
SELECT	Data read karna
DISTINCT	Duplicate values remove
WHERE	Data filter
BETWEEN	Range filter
IN / NOT IN	Multiple values check
UPDATE	Data modify
DELETE	Data remove
Execution Flow	SQL actual run order
GROUP BY + HAVING	Group level filtering

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🟦 INSERT – Table Me Naya Data Add Karna

✔ 1. Simple Insert (All Columns)

INSERT INTO students 
VALUES (1, 'Aman', 95);

✔ 2. Insert Specific Columns

INSERT INTO students (name, marks)
VALUES ('Ravi', 88);

✔ 3. Insert Multiple Rows

INSERT INTO students (id, name, marks)
VALUES 
(3, 'Priya', 90),
(4, 'Vikas', 85),
(5, 'Kiran', 78);

📌 Notes:

• Column order match ho raha ho to VALUES me order same hona chahiye
• Auto Increment columns skip kiye ja sakte hain
• Ek query me multiple rows fast and clean hote hain

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🟦 SELECT – Data Read Karna

✔ All Columns

SELECT * FROM students;

✔ Specific Columns

SELECT name, marks FROM students;

✔ Calculation Ke Sath

SELECT name, marks + 5 AS bonus FROM students;

✔ Unique Values

SELECT DISTINCT city FROM students;

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🟦 WHERE – Row Filtering

WHERE un rows ko choose karta hai jo condition match kare.

✔ Example

SELECT * FROM students
WHERE marks > 80;

Operators

• =, !=, >, <, >=, <=
• AND, OR, NOT
• LIKE
• IS NULL, IS NOT NULL

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🟦 BETWEEN – Range Filtering

SELECT * FROM students
WHERE marks BETWEEN 70 AND 90;

✔ Dono side inclusive
✔ Clean and easy to read

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🟦 IN / NOT IN – Multiple Value Match

✔ IN

SELECT * FROM students
WHERE city IN ('Delhi', 'Mumbai', 'Pune');

✔ NOT IN

SELECT * FROM students
WHERE city NOT IN ('Delhi', 'Pune');

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🟦 UPDATE – Purane Data Me Changes

✔ Basic Update

UPDATE students
SET marks = 95
WHERE id = 3;

✔ Multiple Columns

UPDATE students
SET name = 'Aman', marks = 99
WHERE id = 1;

✔ Calculation Wala Update

UPDATE products
SET price = price + 50
WHERE category = 'Electronics';

✔ Conditional (CASE)

UPDATE students
SET grade = CASE
    WHEN marks >= 90 THEN 'A'
    WHEN marks >= 75 THEN 'B'
    ELSE 'C'
END;

⚠ WHERE na lagane par pura table update ho jata hai

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🟦 DELETE – Table Se Data Remove Karna

✔ Specific Delete

DELETE FROM students WHERE id = 5;

✔ Conditional Delete

DELETE FROM students
WHERE marks < 40;

✔ Delete With Limit (MySQL)

DELETE FROM logs
ORDER BY created_at
LIMIT 10;

⚠ WHERE na lagaya to saara data delete ho jayega

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🧠 SQL Query Execution Flow – Kaise Run Hoti Hai Query?

SQL query likhne ka order kuch aur hota hai…
Par execute hone ka order alag hota hai.

🔥 Actual Execution Order

1. FROM  
2. JOIN  
3. WHERE  
4. GROUP BY  
5. HAVING  
6. SELECT  
7. ORDER BY  
8. LIMIT

Is order ko samajh lena = SQL master ho jana.

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👑 GROUP BY + HAVING – Group Level Filtering

Query:

SELECT city, COUNT(*)
FROM students
WHERE marks > 80
GROUP BY city
HAVING COUNT(*) > 5
ORDER BY COUNT(*) DESC
LIMIT 3;

Step-by-step Explanation:

• FROM → table load hota hai
• WHERE → marks > 80 wale rows bachte hain
• GROUP BY → city ke hisab se groups bante hain
• COUNT(*) → har group me kitni rows hain
• HAVING → group level filter apply hota hai
• ORDER BY → highest count sabse pehle
• LIMIT → top 3 groups

✔ HAVING COUNT(*) kya count karta hai?

• WHERE ke baad bachi rows
• Har group ke andar present rows
• Pure group ki row count

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🎉 Aaj Kya Seekha (Day 4 Summary)

✅ INSERT ke saare tarike

✅ SELECT ka full use

✅ WHERE + BETWEEN + IN/NOT IN

✅ UPDATE (multiple + calculation + CASE)

❌ DELETE safely kaise use hota hai

🎯 SQL Query Execution ka real order

👑 GROUP BY + HAVING full clarity

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