mysql数据库无法创建外键

ALTER TABLE child_table MODIFY column_name datatype;

ALTER TABLE parent_table ADD UNIQUE (column_name);

ALTER TABLE parent_table ENGINE=InnoDB;
ALTER TABLE child_table ENGINE=InnoDB;

ALTER TABLE child_table DROP FOREIGN KEY existing_foreign_key_name;
ALTER TABLE child_table ADD CONSTRAINT new_foreign_key_name FOREIGN KEY (column_name) REFERENCES parent_table(column_name);

GRANT REFERENCES ON parent_table TO 'username'@'host';

-- 创建父表
CREATE TABLE parent_table (
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255) NOT NULL
) ENGINE=InnoDB;

-- 创建子表
CREATE TABLE child_table (
    id INT PRIMARY KEY AUTO_INCREMENT,
    parent_id INT,
    name VARCHAR(255) NOT NULL,
    FOREIGN KEY (parent_id) REFERENCES parent_table(id)
) ENGINE=InnoDB;