触发，删除前

Question

我的触发功能有问题（在删除之前）。 不知何故，我不断收到此错误 表格Teacher正在变异，触发器/功能可能看不到 在“ MIN_TABLE”的第4行 执行触发器“ MIN_TABLE”期间发生错误

CREATE TABLE Teachers(
  id varchar2(10) primary key,
  name varchar2(20) not null,
  phone_no varchar2(15) not null
);

CREATE OR REPLACE TRIGGER min_table
BEFORE DELETE ON Teachers
FOR EACH ROW
DECLARE
  v_count NUMBER;
BEGIN
  SELECT COUNT(*) INTO v_count FROM Teachers;
  IF(v_count = 1) THEN
    RAISE_APPLICATION_ERROR(-20001, 'This table cannot less than one record!');
  END IF; 

END;
/