Add a foregin key to a different table - mysql

Question

I'm trying to add username from my st_accounts column into my table results using mySQL.
st_accounts table
`id` int(11) NOT NULL, //Primary Key
`firstname` varchar(50) NOT NULL,
`lastname` varchar(50) NOT NULL,
`username` varchar(50) NOT NULL,
`password` varchar(255) NOT NULL,
`email` varchar(100) NOT NULL,
`img` text NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
results table
`score_ID` int(11) NOT NULL, //Primary key
`score` int(20) NOT NULL,
`date` date NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

I think you want:
create table accounts (
account_id int auto_increment primary key,
. . .
);
create table results (
result_id int auto_increment primary key,
account_id int not null
score int NOT NULL,
score_date date NOT NULL,
constraint fk_results_accounts foreig key (accounts_id) references accounts(account_id)
) ;
Notes:
Declare the primary key explicitly, not in a comment.
My convention is to name the primary key after the name of the table (in singular) with _id after it.
The foreign key has the same name as the primary key -- self documenting.
I made the integer primary keys auto_increment, so the database can assign unique values.