MySql conditional stop script execution

Question

I need to write SQL script which stops if some values not found in DB. Something like this (pseudo-code):

BEGIN;
...
set @a = (select ... );
if @a IS NULL THEN STOP_WITH_ERROR_AND_ROLLBACK();
...
COMMIT;

Can anybody help to me?

UPDATE: for some reasons, I can't use stored procedures or functions.

UPDATE 2: Note: I don't need explicit rollback. Break of script execution are sufficiently. It automatically rollback changes of not-committed transaction.

Answer 1

    start transaction;

    set @a=(select ...);
     -- whatever you want to execute your code like insert, update

    set @b=(select if(@a is null,'ROLLBACK','COMMIT'));
    -- @b is store **ROLLBACK** if @a is null nither store **COMMIT**

    prepare statement1 from @b;
    -- now statement1 store as @a value
    execute statement1;

i hope it solve your problem.