Nội dung của bài học học sẽ giúp các em biết được các vấn đề thường phải giải quyết trong một bài toán quản lí và sự cần thiết phải có CSDL, vai trò của CSDL trong học tập và cuộc sống, các mức thể hiện CSDL và các yêu cầu cơ bản đối với hệ CSDL. Ở dưới phần nội dung lý thuyết có hai bài tập minh họa để các em củng cố lại bài học.

Tóm tắt lý thuyết

1.1. Bài toán quản lí

  • Để quản lý học sinh trong nhà trường, người ta thường lập các biểu bảng gồm các cột, hàng để chứa các thông tin cần quản lý.
  • Một trong những biểu bảng được thiết lập để lưu trữ thông tin về điểm của học sinh như sau:

Hình 1. Ví dụ hồ sơ lớp

1.2. Các công việc thường gặp khi xử lí thông tin của một tổ chức

a. Tạo lập hồ sơ

Trong tạo lập hồ sơ cần thực hiện các công việc như sau:

  • Xác định chủ thể cần quản lý
    • Ví dụ: Trong bài toán quản lý học sinh trong nhà trường thì chủ thể cần quản lý là học sinh
  • Xác định cấu trúc hồ sơ
  • Thu thập, tập hợp hồ sơ thông tin cần thiết cho hồ sơ từ nhiều nguồn khác nhau và lưu trữ chúng theo đúng cấu trúc đã xác định

b. Cập nhật hồ sơ

  • Sửa chữa hồ sơ
  • Chèn thêm hồ sơ
  • Cần xóa hồ sơ

c. Khai thác hồ sơ

Gồm các công việc như sau:

  • Sắp xếp hồ sơ
  • Tìm kiếm
  • Thống kê
  • Lập báo cáo

1.3. Hệ cơ sở dữ liệu

a. Khái niệm CSDL và hệ quản trị CSDL

  • Khái niệm CSDL: Một cơ sở dữ liệu (Database) là một tập hợp các dữ liệu có liên quan với nhau, chứa thông tin của một tổ chức nào đó (như một trường học, một ngân hàng, một công ti, một nhà máy,…), được lưu trữ trên các thiết bị nhớ để đáp ứng nhu cầu khai thác thông tin của nhiều người dùng với nhiều mục đích khác nhau.
    • Ví dụ trong hình 1 ở trên: Hồ sơ được lưu trữ ở bộ nhớ ngoài của máy tính có thể xem là một CSDL (tạm gọi là CSDL hs).
  • Khái niệm Hệ QTCSDL: Phần mềm cung cấp môi trường thuận lợi và hiệu quả để tạo lập, lưu trữ và khai thác thông tin của CSDL được gọi là hệ quản trị cơ sở dữ liệu (Database Management System).
  • Thuật ngữ “Hệ CSDL” để chỉ một CSDL cùng với hệ QTCSDL quản trị và khai thác CSDL đó.
  • Để lưu trữ và khai thác thông tin bằng máy tính cần phải có:
    • CSDL
    • Hệ QTCSDL
    • Các thiết bị vật lý (máy tính, đĩa cứng, mạng máy tính…)

b. Các mức thể hiện của CSDL

Có 3 mức thể hiện của CSDL:

Hình 2. Các mức thể hiện của CSDL

  • Mức vật lý: cho biết dữ liệu được lưu trữ như thế nào.

Hình 2.1. Mức vật lý của CSDL

Ví dụ: Trong CSDL hs các tệp được lưu trữ trên vùng nhớ nào, dữ liệu về mỗi học sinh chiếm bao nhiêu byte?

  • Mức khái niệm: cho biết dữ liệu nào được lưu trữ trong CSDL và giữa các dữ liệu có quan hệ với nhau như thế nào?

Hình 2.2. Mức khái niệm của CSDL

  • Mức khung nhìn: thể hiện phần CSDL mà người dùng cần khai thác

c. Các yêu cầu cơ bản của hệ CSDL

  • Tính cấu trúc: thông tin trong CSDL được lưu trữ theo một cấu trúc xác định
  • Tính toàn vẹn: Các giá trị dữ liệu được lưu trữ trong CSDL phải thỏa mãn một số ràng buộc tùy thuộc vào hoạt động của tổ chức mà CSDL phản ánh
    • Ví dụ: Để đảm bảo tính toàn vẹn dữ liệu trên cột điểm, sao cho điểm nhập vào thang điểm 10, các điểm của môn học phải đặt ràng buộc giá trị nhập vào: >=0 và <=10
  • Tính nhất quán: Sau những thao tác cập nhật dữ liệu và ngay cả khi có sự cố (phần cứng hay phần mềm) xảy ra trong quá trình cập nhật, dữ liệu trong CSDL phải bảo đảm tính đúng đắn
  • Tính an toàn và bảo mật thông tin: CSDL cần được bảo vệ an toàn, phải ngăn chặn được truy xuất không được phép và phải khôi phục được CSDL khi có sự cố ở phần cứng hay phần mềm. Mỗi nhóm người dùng CSDL có quyền hạn và mục đích sử dụng khác nhau. Cần phải có những nguyên tắc và cơ chế bảo mật khi trao quyền truy xuất dữ liệu cho người dùng
    • Ví dụ về tính an toàn thông tin: Học sinh có thể vào mạng để xem điểm của mình trong CSDL của nhà trường, những hệ thống sẽ ngăn chặn nếu HS cố tình muốn sửa điểm. Hoặc khi điện bị cắt đột ngột máy tính hoặc phần mềm bị hỏng thì hệ thống phải khôi phục được CSDL
    • Ví dụ về tính bảo mật: Hệ thống phải ngăn chặn được mọi truy cập bất hợp pháp đến CSDL
  • Tính độc lập: Vì một CSDL phải phục vụ cho nhiều mục đích khác nhau nên dữ liệu phải độc lập với các ứng dụng, không phụ thuộc vào bài toán cụ thể, đồng thời dữ liệu cũng phải độc lập với phương tiện lưu trữ và xử lí
  • Tính không dư thừa:
    • Ví dụ: Một CSDL đã có cột ngày sinh, thì không cần có cột tuổi. Vì năm sau thì tuổi sẽ khác đi, trong khi giá trị của tuổi lại không được cập nhật tự động vì thế nếu không sửa chữa số tuổi cho phù hợp thì dẫn đến tuổi và năm sinh thiếu tính nhất quán

d. Một số ứng dụng có sử dụng CSDL

  • Cơ sở giáo dục: Quản lí học sinh
  • Cơ sở kinh doanh: Quản lí việc mua bán hàng
  • Cơ sở sản xuất: Quản lí dây chuyền sản xuất
  • Tổ chức tài chính: Quản lí tài chính

231 BÌNH LUẬN

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