ALTER TABLE add约束

Question

已正确创建用户表和属性

CREATE TABLE Properties
(
    ID int AUTO_INCREMENT,
    language int,
    stonecolor int,
    gamefield int,
    UserID int,
    PRIMARY KEY(ID),
    FOREIGN KEY(language) REFERENCES Language(ID),
    FOREIGN KEY(stonecolor) REFERENCES StoneColor(ID),
    FOREIGN KEY(gamefield) REFERENCES GameField(ID)
) ENGINE = INNODB;

CREATE TABLE User
(
    ID int AUTO_INCREMENT,
    vorname varchar(30) NOT NULL,
    name varchar(30) NOT NULL,
    email varchar(40) NOT NULL,
    password varchar(40) NOT NULL,
    nickname varchar(15) NOT NULL,
    score int,
    isadmin int DEFAULT 0,
    gamesPlayed int,
    properties int NOT NULL,
    PRIMARY KEY(ID),
    UNIQUE (email),
    UNIQUE (nickname)

) ENGINE = INNODB;

但是，ALTER TABLE User不起作用。

ALTER TABLE User 
(
    ADD CONSTRAINT userPropertie
    FOREIGN KEY(properties)
    REFERENCES Properties(ID)
)

不知道为什么？

我用这个作为参考http://www.w3schools.com/sql/sql_foreignkey.asp

错误1064-您的SQL语法中有一个错误；检查与您的MySQL服务器版本对应的手册，以获得正确的语法使用在“（添加约束用户属性FOREIGN KEY（属性）REFERENCES属性（'在第2行

Answer 1

alter table User 
    add constraint userProperties
    foreign key (properties)
    references Properties(ID)

Answer 2

ALTER TABLE `User`
ADD CONSTRAINT `user_properties_foreign`
FOREIGN KEY (`properties`)
REFERENCES `Properties` (`ID`)
ON DELETE NO ACTION
ON UPDATE NO ACTION;

Answer 3

省略括号：

ALTER TABLE User 
    ADD CONSTRAINT userProperties
    FOREIGN KEY(properties)
    REFERENCES Properties(ID)