mysql语句约束

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL
);

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    username VARCHAR(50) NOT NULL
);

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) UNIQUE
);

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    user_id INT,
    order_date DATE,
    FOREIGN KEY (user_id) REFERENCES users(user_id)
);

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    age INT CHECK (age >= 0 AND age <= 100)
);

INSERT INTO users (user_id, username) VALUES (1, 'john'); -- 如果user_id=1已存在，会报错

INSERT INTO users (user_id, email) VALUES (2, 'john@example.com'); -- 如果email已存在，会报错

INSERT INTO orders (order_id, user_id, order_date) VALUES (1, 3, '2023-10-01'); -- 如果user_id=3在users表中不存在，会报错

INSERT INTO users (user_id, username, age) VALUES (3, 'jane', 150); -- 如果age不在0到100之间，会报错