mysql里怎么写约束

CREATE TABLE students (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

CREATE TABLE students (
    id INT AUTO_INCREMENT PRIMARY KEY,
    email VARCHAR(100) UNIQUE
);

CREATE TABLE students (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

CREATE TABLE students (
    id INT AUTO_INCREMENT PRIMARY KEY,
    status VARCHAR(50) DEFAULT 'active'
);

CREATE TABLE students (
    id INT AUTO_INCREMENT PRIMARY KEY,
    class_id INT,
    FOREIGN KEY (class_id) REFERENCES classes(id)
);

CREATE TABLE classes (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100)
);

DELIMITER $$
CREATE TRIGGER check_age
BEFORE INSERT ON students
FOR EACH ROW
BEGIN
    IF NEW.age < 0 THEN
        SIGNAL SQLSTATE '45000'
        SET MESSAGE_TEXT = 'Age cannot be negative';
    END IF;
END$$
DELIMITER ;