Constraints
NOT NULL, ENSURE, karta hai, ki, EK, column, mein, HAMESHA, EK, VALUE, ho, EMPTY, nahi. UNIQUE, DUPLICATE, values, ko, EK, column, mein, PREVENT, karta hai (jaise, email).
CHECK, constraint, (MySQL 8.0.16+) EK, CUSTOM, condition, ENFORCE, karta hai (jaise, age >= 18). DEFAULT, EK, column, ke, liye, EK, DEFAULT, value, SET, karta hai, agar, EXPLICITLY, VALUE, na, di, jaaye.
CREATE TABLE users (
id INT PRIMARY KEY AUTO_INCREMENT,
email VARCHAR(100) NOT NULL UNIQUE,
age INT CHECK (age >= 18),
status VARCHAR(20) DEFAULT 'active'
);- NOT NULL = value, ZAROORI, hai. UNIQUE = duplicate, values, nahi, ho, sakte
- CHECK = custom condition, enforce, karta hai
- Constraints, application-level, validation, se, MORE, reliable, HAIN
UNIQUE, constraint, MULTIPLE, columns, PAR, bhi, apply, ki, ja, sakti, hai — jaise, (user_id, product_id), COMBINATION, unique, hona, chahiye, EK, wishlist, table, mein, — individual, columns, unique, nahi, hain, LEKIN, combination, hai.
CREATE TABLE wishlist (
user_id INT, product_id INT,
UNIQUE KEY unique_wishlist (user_id, product_id)
);ALTER TABLE, se, EXISTING, table, mein, constraints, ADD, kiye, ja, sakte, hain — LEKIN, agar, EXISTING, data, NAYE, constraint, ko, VIOLATE, karta, hai, (jaise, duplicate, values, UNIQUE, constraint, ke, liye), ALTER, statement, FAIL, ho, jaayegi.