mysql怎么设置唯一性约束条件

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

ALTER TABLE users
ADD CONSTRAINT unique_email UNIQUE (email);

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');
INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com'); -- 这将失败

ALTER TABLE users
DROP INDEX unique_email;

START TRANSACTION;
INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');
-- 如果插入失败，捕获错误并回滚事务
IF @@error_count > 0 THEN
    ROLLBACK;
ELSE
    COMMIT;
END IF;