SQL - If id exists in table1, then insert record into table2

Question

I have the following problem. I have an accounts and a scoreboard table and I try to reset all values on the end of the month in the scoreboard by inserting a new record with the value 0.

SET @mid = (SELECT MAX(id) AS mid FROM accounts);

WHILE (@mid > 0)
BEGIN
    IF @mid NOT IN (SELECT id FROM accounts) DO CONTINUE;
    INSERT INTO scoreboard("uid", "balance", "date") VALUES (@mid, 0,(SELECT CURRENT_DATE()));
    SET @mid = @mid - 1;
END

But whatever I try it breaks and I have no idea why.

MySQL: mysqlnd 5.0.12-dev - 20150407

MariaDB: 10.3.27-MariaDB-0+deb10u1

Example:

accounts

id	name
1	Play_it
3	User2

scoreboard

uid	balance	date
1	20	2021-03-09

Expected:

scoreboard

uid	balance	date
1	20	2021-03-09
1	40	2021-03-31
3	30	2021-03-31
1	0	2021-04-01
3	0	2021-04-01

Answer 1

Do you want something like this?

insert into scoreboard (uid, balance, date)
    select a.id, 0, curdate()
    from accounts a;

This inserts a new row in scoreboard for each row in account .