3.使用MySQL 语句在 salary_management_system数据库中创建4个数据表 tbl_departments、tbl_rank_ salary、tbl_employees、tb_salary。

在MySQL中创建这些表格的SQL语句会分别如下所示：

1. 创建 tbl_departments (部门) 表格：

CREATE TABLE tbl_departments (
    id INT AUTO_INCREMENT PRIMARY KEY,
    department_name VARCHAR(50) NOT NULL
);

2. 创建 tbl_ranks_salary (职位工资等级) 表格：

CREATE TABLE tbl_ranks_salary (
    rank_id INT AUTO_INCREMENT PRIMARY KEY,
    rank_name VARCHAR(50),
    min_salary DECIMAL(10, 2),
    max_salary DECIMAL(10, 2)
);

3. 创建 tbl_employees (员工) 表格：

CREATE TABLE tbl_employees (
    employee_id INT AUTO_INCREMENT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    department_id INT,
    rank_id INT,
    FOREIGN KEY (department_id) REFERENCES tbl_departments(id),
    FOREIGN KEY (rank_id) REFERENCES tbl_ranks_salary(rank_id)
);

4. 创建 tb_salary (员工薪资) 表格（假设薪资记录包含时间信息，如月度薪资）：

CREATE TABLE tb_salary (
    salary_id INT AUTO_INCREMENT PRIMARY KEY,
    employee_id INT,
    salary_amount DECIMAL(10, 2),
    month DATE,
    FOREIGN KEY (employee_id) REFERENCES tbl_employees(employee_id)
);