delimiter //
create procedure setBooks()
begin
	set @i = 1;
	loop1: loop
		set @authorId = (
			select id from authors order by rand() limit 1
		);
        set @themeId = (
			select id from themes order by rand() limit 1
        );
		insert into books (
			name, pages, price, publishDate, authorId, themeId
        ) values (
			concat('book', @i), floor(rand() * 500) + 20,
            rand() * 500000 / 100 + 100,
            current_timestamp(), @authorId, @themeId
        );
        set @i = @i + 1;
        if @i > 500 then leave loop1; end if;
    end loop loop1;
end//
delimiter ;

call setBooks();