I'm doing a fair amount of guessing here, but this is how I imagine that your model should look like. In a couple of tables I removed id columns to make it clearer, you may want to put them back in case the keys I have chosen is not stable enough:

    CREATE TABLE `users` (
      `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
      `name` varchar(100),
      `password` varchar(255) NOT NULL,
       PRIMARY KEY (`id`),
       UNIQUE(`name`) -- questionable
    );

    insert into users(name, password) values ('bob', 'passwd');

    CREATE TABLE `stock` (
        stock_code varchar(100) COLLATE utf8_unicode_ci NOT NULL,
        PRIMARY KEY (stock_code)
    );

    insert into stock(stock_code) values('AABC'), ('BBCC'), ('EEFF');

    -- a user can have 0 or more stocks, a stock have 1 user 
    create table user_stock (
        user_id int(10) unsigned NOT NULL,
        stock_code varchar(100) COLLATE utf8_unicode_ci NOT NULL,
        `value` double DEFAULT NULL,
        PRIMARY KEY (stock_code),
        constraint fk1_user_stock foreign key (user_id)
            references `users` (id)
                on delete cascade
                on update cascade,
        constraint fk2_user_stock foreign key (stock_code)
            references stock (stock_code)
                on delete cascade
                on update cascade
    );

    insert into user_stock(stock_code, value, user_id) 
    values('AABC', 10, 1), ('BBCC', 4.5, 1), ('EEFF', 7, 1);

    CREATE TABLE `sector_a` (
        `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
        `name` varchar(100) CHARACTER SET utf8 DEFAULT NULL,
        PRIMARY KEY (`id`),
        UNIQUE(name)
    );

    insert into sector_a(name) values('FOOD'), ('CAR'), ('BUILDING');

    -- a stock can belong to 0 or more sectors, a sector can have 0 or more stocks
    CREATE TABLE `stock_sector_a` (
        stock_code varchar(100) COLLATE utf8_unicode_ci NOT NULL,
        `sector_id` int(10) unsigned not NULL,
        PRIMARY KEY (stock_code, sector_id),
        constraint fk1_stock_sector_a foreign key (stock_code)
            references stock (stock_code)
                on delete cascade
                on update cascade,
        constraint fk2_stock_sector_a foreign key (sector_id)
            references sector_a (id)
                on delete cascade
                on update cascade
    );

    insert into stock_sector_a(stock_code, sector_id)
    values('AABC', 1), ('AABC', 2), ('AABC', 3), ('BBCC', 2), ('BBCC', 3);

    MariaDB [test]> delete from stock where stock_code = 'AABC';
    Query OK, 1 row affected (0.01 sec)

    MariaDB [test]> select * from user_stock;
    +---------+------------+-------+
    | user_id | stock_code | value |
    +---------+------------+-------+
    |       1 | BBCC       |   4.5 |
    |       1 | EEFF       |     7 |
    +---------+------------+-------+
    2 rows in set (0.00 sec)

    MariaDB [test]> select * from stock_sector_a;
    +------------+-----------+
    | stock_code | sector_id |
    +------------+-----------+
    | BBCC       |         2 |
    | BBCC       |         3 |
    +------------+-----------+
    2 rows in set (0.00 sec)