如何在现有表中添加外键约束

Question

我有2个表首先是smsusers(id,fname,lname,userid) 'id'是主键, userid是唯一键

address_detail(id,address,type);

在这里我要补充userid的smsusers是外键address_detail表.

因为我添加了一列address_detail作为user_id

ALTER TABLE address_detail add user_id INT( 20 )  default NULL;

现在我正在向表中添加外键约束

ALTER TABLE `address_detail` add CONSTRAINT address_detail_fk FOREIGN KEY (user_id) 
REFERENCES smsusers ('userid') ON UPDATE CASCADE ON DELETE CASCADE;

此步骤显示以下错误

#1064 - You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server
version for the right syntax to use near ''userid') ON 
UPDATE CASCADE ON DELETE CASCADE' at line 1

Answer 1

我会删除'查询中的on userid

ALTER TABLE `address_detail` add CONSTRAINT address_detail_fk FOREIGN KEY (user_id) 
REFERENCES smsusers (userid) ON UPDATE CASCADE ON DELETE CASCADE;