Lesson 02 � Foundation

DATA NIKALO
TABLE SE.

SELECT SQL ka sabse zyada use hone wala command hai. Chahe aapko kuch columns chahiye, saara data chahiye, ya specific rows � sab kuch SELECT se start hota hai. Yeh lesson aapko sikhayega ki data ko table se kaise nikalte hain.

WHY: SELECT itna important kyun hai?

Socho aapke paas ek students table hai jisme 1000 students ka data hai. Agar aapko sirf names aur marks chahiye, toh poori table nahi dekhni � sirf woh columns nikalne hai jo aapko chahiye. SELECT se aap exactly choose kar sakte ho ki kaunsa data dikhana hai.

HOW: SELECT ki basic syntax

-- Specific columns nikalo
SELECT name, marks FROM students;

-- Saara data nikalo
SELECT * FROM students;

Pehle SELECT likho, phir column names do (comma se separate), phir FROM aur table ka naam. Agar saara chahiye toh * use karo.

Alias: naam badlo

Output mein column ka naam padhne mein accha lage � isliye alias use karte hain. AS keyword se column ka temporary naam badal sakte ho.

-- Column alias
SELECT name AS StudentName, marks AS Score FROM students;

-- Calculation ke saath alias
SELECT name, marks, marks * 10 AS percentage FROM students;

Yahan name ki jagah output mein StudentName dikhega aur marks * 10 ka result percentage naam se aayega.

DISTINCT: unique values nikalo

Agar aapko pata karna hai ki kitne alag-alag courses hain, toh DISTINCT use karo. Yeh duplicate values hata deta hai.

-- Unique courses
SELECT DISTINCT course FROM students;

WHERE: basic filter

Abhi aapne poora data dekha. Lekin agar sirf high-scoring students chahiye? WHERE clause se condition lagao.

-- Marks 80 se zyada wale students
SELECT * FROM students WHERE marks > 80;

-- Specific course
SELECT name, marks FROM students WHERE course = 'Data Science';

LIMIT: kitne rows chahiye

Agar sirf top 5 students dekhne hain, toh LIMIT use karo. Yeh large datasets mein bahut kaam aata hai.

-- Sirf 5 rows
SELECT * FROM students LIMIT 5;

-- Top 3 marks wale students
SELECT name, marks FROM students ORDER BY marks DESC LIMIT 3;

Common mistakes

• SELECT name marks FROM students galat hai � columns ke beech comma lagao: SELECT name, marks.
• SELECT FROM students invalid hai � SELECT ke baad column name ya * hona chahiye.
• select aur SELECT dono kaam karte hain (SQL case-insensitive hai), lekin convention uppercase mein likhna hai.
• SELECT * FROM students WHERE marks = "92" galat hai � numbers ke quotes nahi lagte.
• LIMIT hamesha end mein aata hai, WHERE ke baad.