How to organize a relationship in tables one to one mysql?

Question

MySQL

How to organize a one-to-one connection through the console. There are two tables Groups and Users.
There are 3 columns in the Groups table:
1. id
2. id_group
3. name
There are 3 columns in the Users table:
1. id
2. name_user
3. groups
there are 3 groups (1 - Easy, 2 - Medium, 3 - Hard)
if in the User table the groups column is given the number 2 to substitute from the Groups table name of the standing id_group.
I try like this

ALTER TABLE `groups` ADD FOREIGN KEY ( `name` ) REFERENCES `users` (`groups`) ON DELETE RESTRICT ON UPDATE RESTRICT ;

but gives out

#1170 - BLOB column 'name' was specified in the key definition without specifying the key length

Answer 1

You are doing complete nonsense. Linking by text field is fundamentally not the right solution. In addition, I suspect that users.groups does not contain the name of one group, but a comma-separated list of groups! And this is generally game!
Why don't you have a group_id field and link users.group_id to groups.id?