1. CREATE TABLE employee(
    2.   emp_id INT PRIMARY KEY,
    3.   first_name VARCHAR(20),
    4.   last_name VARCHAR(20),
    5.   birth_day DATE,
    6.   sex VARCHAR(1),
    7.   salary INT,
    8.   super_id INT,
    9.   branch_id INT
    10. )
    12. # 联级删除
    13. CREATE TABLE branch(
    14.   branch_id INT PRIMARY KEY,
    15.   branch_name VARCHAR(40),
    16.   mgr_id INT.
    17.   mgr_start_date DATE,
    18.   FOREIGN KEY(mgr_id) REFERENCES employee(emp_id) ON DELETE CASCADE
    19. )
    21. # 删除后设置为NULL
    22. CREATE TABLE client(
    23.   client_id INT PRIMARY KEY,
    24.   client_name VARCHAR(40),
    25.   branch_id INT,
    26.   FOREIGN KEY(branch_id) REFERENCES branch(branch_id) ON DELETE SET NULL
    27. )