Lesson 06 � Modify

DATA BADLO,
TABLE BHI BADLO.

Ek baar data daal diya toh kya hamesha wahi rahega✓ Bilkul nahi! Marks update karo, address change karo, naya column add karo � sab UPDATE aur ALTER se hota hai. Bina WHERE ke poora table update ho jaata hai, isliye dhyan se seekho!

WHY: UPDATE kyun zaruri hai?

Data fix karna padta hai � student ke marks galat daal diye, address badal gaya, course upgrade karna hai. Ye sab manually nahi kar sakte, UPDATE se ek line mein ho jaata hai. Aur jab table ki structure badalni ho � naya column add karna hai, purana hatana hai � toh ALTER TABLE use hota hai.

HOW: UPDATE ka syntax

UPDATE mein pehle table ka naam aata hai, phir SET keyword se column-value pairs, aur optional WHERE clause se row select karte ho.

1. Single column update

Ek student ke marks 90 set karo:

UPDATE students SET marks = 90.0 WHERE id = 1;

2. Multiple columns update

Aman ke marks aur course dono change karo:

UPDATE students SET marks = 95.0, course = 'Advanced Python' WHERE name = 'Aman';

Columns ke beech comma lagao � jaise SELECT mein column names dete ho.

3. All rows update (DANGER!)

Sabke marks zero kar do � WHERE nahi lagaya toh poora table badal gaya:

UPDATE students SET marks = 0;

4. Update with calculation

Sab Python students ke marks mein 5 bonus do:

UPDATE students SET marks = marks + 5 WHERE course = 'Python';

Existing value ka expression bana sakte ho � marks + 5 ya marks * 1.1 (10% increase).

5. Update with NULL

Kisi ka address clear karo:

UPDATE students SET address = NULL WHERE id = 3;

ALTER TABLE: structure badlo

UPDATE se data badalta hai, ALTER se table ka structure � naya column add karna, column ka type change karna, ya column hatana.

1. Column add karo

Students table mein email column jodo:

ALTER TABLE students ADD email VARCHAR(100);

2. Column modify karo

Age column ka type VARCHAR mein badlo:

ALTER TABLE students MODIFY age VARCHAR(10);

3. Column drop karo

Email column hata do:

ALTER TABLE students DROP COLUMN email;

4. Column rename karo (MySQL)

ALTER TABLE students RENAME COLUMN marks TO score;

Try it: student records update karo

Neeche diye gaye SQL queries ko editor mein modify karo aur dekho output kaise badalta hai. Pehle single row update try karo, phir multiple columns.

Common beginner mistakes

• WHERE bhoolna: UPDATE students SET marks = 0 � poora table zero ho jaata hai! Hamesha WHERE lagao jab specific row target karo.
• Single quotes numbers pe: SET marks = '95' galat hai � numbers ke quotes nahi lagte. Sirf strings pe lagao: SET name = 'Aman'.
• Column name galat: SET mark = 95 agar column ka naam marks hai toh error aayega. Column name exactly woh hona chahiye jo table mein hai.
• ALTER mein typo: ALTER TABL students � TABLE spell check karo. SQL keywords case-insensitive hain lekin galat spelling se error aata hai.
• Comma bhoolna: SET marks = 95 course = 'Python' galat hai � columns ke beech comma lagao: SET marks = 95, course = 'Python'.