我不断得到：
错误代码：1451，不能删除或更新父行：外键约束在MySQL上失败。
它只出现在“如果存在分支则删除表”和“如果存在人员则删除表”上，但其余部分功能正常。

CREATE DATABASE IF NOT EXISTS Optician;
USE Optician;

DROP TABLE IF EXISTS branches;
CREATE TABLE branches (
  branch_id INT PRIMARY KEY,
  branch_name VARCHAR(255) NOT NULL,
  branch_address ENUM('Cardiff', 'Swansea', 'Newport') NOT NULL,
  staff_available BOOLEAN NOT NULL
);

DROP TABLE IF EXISTS staff;
CREATE TABLE staff (
  staff_id INT PRIMARY KEY,
  staff_name VARCHAR(255) NOT NULL,
  staff_role ENUM('Optician', 'Receptionist', 'Technician') NOT NULL,
  staff_available BOOLEAN DEFAULT true,
  welsh_speaker BOOLEAN DEFAULT false,
  branch_id INT,
  FOREIGN KEY (branch_id) 
  REFERENCES branches(branch_id) ON DELETE CASCADE
);

DROP TABLE IF EXISTS appointments;
CREATE TABLE appointments (
  appointment_id INT PRIMARY KEY,
  customer_name VARCHAR(255) NOT NULL,
  appointment_type VARCHAR(255) NOT NULL,
  appointment_date DATE NOT NULL,
  appointment_time TIME NOT NULL,
  staff_id INT NOT NULL,
  FOREIGN KEY (staff_id) 
  REFERENCES staff(staff_id) ON DELETE CASCADE,
  UNIQUE (appointment_date, appointment_time, staff_id)
);

DROP TABLE IF EXISTS orders;
CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_name VARCHAR(255) NOT NULL,
  customer_phone VARCHAR(225) NOT NULL,
  customer_email VARCHAR(255) NOT NULL,
  order_type VARCHAR(255) NOT NULL,
  order_status VARCHAR(255) NOT NULL,
  order_date DATE NOT NULL,
  technician_id INT NOT NULL,
  lens_arrival_date DATE,
  assembly_start_date DATE,
  assembly_completion_date DATE,
  FOREIGN KEY (technician_id) 
  REFERENCES staff(staff_id)
);