mysql 怎么约束性别

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    gender ENUM('男', '女') NOT NULL
);

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    gender CHAR(1) NOT NULL,
    CHECK (gender IN ('男', '女'))
);

CREATE TABLE genders (
    id INT PRIMARY KEY,
    name VARCHAR(255) NOT NULL UNIQUE
);

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    gender_id INT NOT NULL,
    FOREIGN KEY (gender_id) REFERENCES genders(id)
);

INSERT INTO genders (name) VALUES ('男'), ('女');

INSERT INTO users (name, gender_id) VALUES ('张三', 1), ('李四', 2);

INSERT INTO users (name, gender) VALUES ('王五', '未知'); -- 会报错

ALTER TABLE users MODIFY COLUMN gender ENUM('男', '女', '未知') NOT NULL;