SQL-Procedures: NOT EXISTS seems to be always true

Question

Hello stackoverflow members.

First, I'm new to SQL and have almost only theory knowledge about databases. This is the table:

create table ACCOUNT (LOGIN varchar (16) primary key, MAIN boolean not null);

Now I have created this procedure for inserting a new tuple:

delimiter $$
create procedure add_account (in login varchar (16))
    begin
        declare main boolean;
        set main = not exists (select LOGIN from ACCOUNT where MAIN);
        insert into ACCOUNT (LOGIN, MAIN) values (login, main);
    end
$$
delimiter ;

My intention: By using add_account, only one tuple in ACCOUNT has the value true for attribute MAIN. But after that, it seems that the NOT EXISTS is always true:

call add_account ('First');
call add_account ('Second');
select * from ACCOUNT;
+--------+------+
| LOGIN  | MAIN |
+--------+------+
| First  |    1 |
| Second |    1 |
+--------+------+

The second should be 0 because there is already a tuple that is 1. Anyone got ideas? What is wrong about my code/thinking?

Thanks in advance and Greetings

---

After Gordon's answer, I renamed the lower-case main to foo. My wrong thinking: I thought that lower-case main would be seen as different from upper-case MAIN. Now the results are as I wished:

call add_account ('First');
call add_account ('Second');
select * from ACCOUNT;
+--------+------+
| LOGIN  | MAIN |
+--------+------+
| First  |    1 |
| Second |    0 |
+--------+------+

Answer 1

Let me explain this logic, line by line:

1. declare main boolean;
2. set main = not exists (select LOGIN from ACCOUNT where MAIN);
3. insert into ACCOUNT (LOGIN, MAIN) values (login, main);

Line 1 declares a variable called main as a boolean. By default, the value will be NULL, which is treated as false.

Line 2 runs the following query:

select LOGIN
from ACCOUNT
where MAIN

Well, MAIN is NULL -- basically "false" -- hence this returns no values. So, the NOT EXISTS returns "true", which is then assigned to main

Line 3 inserts the new value in the row.

This is clearly not your intention. I would suggest that you ask another question and describe what you want to do, along with sample data and desired results.