DELETE命令需要 FROM。

The DELETE command requires a FROM.

要访问触发表中的值，您可以必须使用旧或新别名。

To access values in the triggering table, you have to use the OLD or NEW aliases.

CREATE TRIGGER Delete_B_trigger
AFTER DELETE ON A
FOR EACH ROW
BEGIN
    DELETE FROM B WHERE fid = OLD.id;
END