无法设置唯一键以避免插入重复行？

-- 示例：假设表名为 users，列名为 email
CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(255) NOT NULL UNIQUE
);

-- 示例：删除重复的 email
DELETE FROM users
WHERE id NOT IN (SELECT MIN(id) FROM users GROUP BY email);

-- 示例：删除现有的唯一键
ALTER TABLE users DROP INDEX email_unique;

-- 示例：授予权限
GRANT INDEX ON users TO 'username'@'localhost';

-- 示例：更改数据库引擎
ALTER TABLE users ENGINE=InnoDB;

-- 创建表
CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(255) NOT NULL UNIQUE
) ENGINE=InnoDB;

-- 插入数据
INSERT INTO users (id, email) VALUES (1, 'user1@example.com');
INSERT INTO users (id, email) VALUES (2, 'user2@example.com');

-- 尝试插入重复数据（会失败）
INSERT INTO users (id, email) VALUES (3, 'user1@example.com');

-- 删除重复数据
DELETE FROM users
WHERE id NOT IN (SELECT MIN(id) FROM users GROUP BY email);

-- 再次尝试插入重复数据（成功）
INSERT INTO users (id, email) VALUES (3, 'user1@example.com');