将varchar列更改为另一个表SQL的外键

Question

I'm trying to change a column's varchar values to it's id foreign key value from another table, and I can't seem to make the conversion, always throws an error.

User table...

CREATE TABLE User(
UserId INT NOT NULL AUTO_INCREMENT,
FirstName VARCHAR(50),
LastName VARCHAR(50),
Email VARCHAR(50),
Username VARCHAR(20) NOT NULL,
Password VARCHAR(50) NOT NULL,
RoleName VARCHAR(50) NOT NULL,
LastLogin DATETIME,

PRIMARY KEY(UserId)
);

INSERT INTO User 
 (FirstName, LastName, Email, Username, Password, RoleName)
VALUES 

...

RoleName table...

create table rolename(
                RoleId int not null auto_increment,
                RoleName varchar(50),
                primary key(RoleId)
);
insert into rolename(RoleName) select distinct RoleName from user;

Now, what I need to do is have the RoleName column in the User table point to the key in the RoleName table. I keep trying this, but always gives me an error.

update user set user.`RoleName` = rolename.`RoleId` from user inner join rolename on user.`RoleName` = rolename.`RoleName`;

I'm not sure if I'm setting up the foreign key properly with that code either...

Answer 1

No, you are doing this wrong. You put the id in the User table. To fix this:

alter table user add roleid int;
alter table user add constraint fk_user_roleid foreign key (roleid)  references rolename(roleid);

Then populate the values:

update user u join
       rolename r
       on r.rolename = u.rolename
    set u.roleid = r.roleid;

And get rid of the old column!

alter table user drop column rolename;