mysql - How to add a foreign key constraint in existing table

Question

I am having 2 tables first is smsusers(id,fname,lname,userid) 'id' is primary key, userid is unique key

address_detail(id,address,type);

Here I want to add userid of smsusers as foreign key to address_detail table.

for that i added a column to address_detail as user_id

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

Now I am adding foreign key constraint to table

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

This step is showing following error

#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'在查询中删除 on userid

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

Answer 2

应该有反引号。尝试这个：

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

Answer 3

该REFERENCES子句需要括号之间的列名，但您提供了一个字符串（它恰好是一个列名，但这无关紧要）。删除引号：

....
REFERENCES smsusers (userid) -- no quotes
....