若要向HTML表格中的嵌套数组列插入标题，可以按如下所示修改代码：

const Table = college?.map((item) => {
  return (
    <>
      <tr>
        <th colSpan="3">{item.departmentName}</th>
      </tr>
      <tr>
        <th>Student Name</th>
        <th>Subject</th>
        <th>Days Attended Classes</th>
      </tr>
      {item.department.map((student, i) => (
        <tr key={i}>
          <td>{student.studentName}</td>
          <td>{student.subject}</td>
          <td>{student.daysClassattended}</td>
        </tr>
      ))}
    </>
  );
});

return (
  <div>
    <table>
      <tbody>{Table}</tbody>
    </table>
  </div>
);

如果您只是想列出所有学生而不区分他们的系，那么flatMap是一个选项：

const Table = college?.flatMap((item =>   
    item.department.map((item,i) => (
      <tr>
        <td>{item.studentName}</td>
        <td>{item.subject}</td>
        <td>{item.daysClassattended}</td>
      </tr>                        
    ))
  ));