Data organisation and data control

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教育者の方へ： Data organisation and data control（Information and Communication Technology、Information Processing）向けのすぐ使えるレッスンスライド, 復習ノート — レッスンで使うか、学習者がライブゲームとして遊ぶインタラクティブなクラス活動としてトピックを実施できます。

レッスンノート

The data hierarchy 數據層級

Data is organised in a hierarchy from smallest to largest: character → field → record → file → database.
數據按層級由小至大組織：字元 → 欄位 → 記錄 → 檔案 → 數據庫。
Character: a single letter, digit or symbol, e.g. 'A', '7', '@'. Field: one item of data about something, e.g. a student's name.
字元：單一字母、數字或符號，例如「A」、「7」、「@」。欄位：關於某事物的一項數據，例如學生姓名。
Record: a set of related fields about one person or thing, e.g. all the details of one student.
記錄：關於一個人或一件事物的一組相關欄位，例如一名學生的全部資料。
File (or table): a collection of related records, e.g. all students in the school. Database: a collection of related files, e.g. student, subject and fee files.
檔案（或資料表）：一組相關記錄，例如全校學生。數據庫：一組相關檔案，例如學生、科目及學費檔案。

Key fields 關鍵欄位

A key field (primary key) holds a value that is unique for every record, so each record can be identified.
關鍵欄位（主鍵）的值對每一個記錄都是獨一無二的，因此可以識別每個記錄。
Good key fields: student ID, HKID number, product code. Poor key fields: name, class, date of birth — these can repeat.
良好的關鍵欄位：學生編號、身份證號碼、貨品編號。不良的關鍵欄位：姓名、班別、出生日期——這些可能重複。
Records are often sorted by the key field so that they can be found quickly.
記錄通常會按關鍵欄位排序，以便快速找到。

Sequential access 順序存取

In sequential access, records are read one after another from the beginning until the required record is found.
在順序存取中，記錄由開頭逐一讀取，直至找到所需記錄。
Example medium: magnetic tape. Used when most records are processed each time, e.g. monthly payroll, utility bills, backups.
例子媒體：磁帶。適用於每次處理大部分記錄的情況，例如每月計算薪金、水電費單、備份。
Advantages: simple, cheap, efficient when all records are processed in order (batch processing).
優點：簡單、便宜，當按次序處理所有記錄時很有效率（批次處理）。
Disadvantages: slow to find a single record; inserting or deleting a record usually means rewriting the file.
缺點：尋找單一記錄很慢；插入或刪除記錄通常需要重寫整個檔案。

Direct (random) access 直接（隨機）存取

In direct access, the system goes straight to the required record without reading the ones before it (using an address or an index).
在直接存取中，系統直接前往所需記錄而不需讀取之前的記錄（利用位址或索引）。
Example media: hard disk, SSD, flash memory. Used when individual records are needed quickly, e.g. ATM balance enquiry, airline booking, library catalogue.
例子媒體：硬碟、固態硬碟、快閃記憶體。適用於需要快速取得個別記錄的情況，例如自動櫃員機查詢餘額、機票訂位、圖書館目錄。
Advantages: fast retrieval of any record; easy to update, insert or delete one record.
優點：可快速讀取任何記錄；易於更新、插入或刪除單一記錄。
Disadvantages: more complex and more expensive storage; less efficient if every record must be processed in order.
缺點：較複雜及儲存成本較高；若須按次序處理所有記錄則效率較低。

Why data needs controlling 為何需要控制數據

"Garbage in, garbage out" (GIGO): if wrong data is entered, the output will also be wrong, however good the program is.
「垃圾入，垃圾出」：若輸入錯誤數據，無論程式多好，輸出亦會是錯的。
Errors come from typing mistakes, transcription errors (copying wrongly), transposition errors (e.g. 5162 typed as 5612) and transmission errors (noise changes bits).
錯誤來自打字錯誤、抄寫錯誤、換位錯誤（例如 5162 打成 5612）及傳送錯誤（雜訊令位元改變）。
Data control aims to prevent errors (verification) and detect errors (validation, parity check).
數據控制旨在防止錯誤（核實）及偵測錯誤（有效性檢驗、奇偶校驗）。

Validation 有效性檢驗

Validation is an automatic check by the computer that data is reasonable and follows the rules. It cannot prove data is correct.
有效性檢驗是電腦自動檢查數據是否合理及符合規則，但不能證明數據正確。
Range check: value within limits, e.g. mark 0–100. Type check: correct data type, e.g. age must be a number.
範圍檢查：數值在限制之內，例如分數 0–100。類型檢查：數據類型正確，例如年齡必須是數字。
Length check: correct number of characters, e.g. HK phone number has 8 digits. Presence check: a required field is not left empty.
長度檢查：字元數目正確，例如香港電話號碼有 8 位數字。存在檢查：必填欄位不可留空。
Format check: data matches a pattern, e.g. date DD/MM/YYYY. Check digit: an extra digit calculated from the others, e.g. the digit in brackets on an HKID card or an ISBN.
格式檢查：數據符合某個格式，例如日期 DD/MM/YYYY。核對數位：由其他數字計算出的額外數字，例如身份證括號內的數字或 ISBN。
A mark of 67 entered as 76 passes a range check — validation cannot catch every error.
把分數 67 輸入成 76 仍會通過範圍檢查——有效性檢驗不能找出所有錯誤。

Verification 核實

Verification checks that data has been copied or entered exactly as in the original source.
核實是檢查數據是否與原始來源完全一致地抄寫或輸入。
Double entry: data is entered twice (e.g. a new password typed twice) and the two copies are compared.
雙重輸入：數據輸入兩次（例如新密碼輸入兩次），然後比較兩份是否相同。
Visual check (proofreading): a person compares the entered data with the source document on screen.
目視檢查（校對）：由人把已輸入的數據與原始文件在螢幕上比較。
Verification does not check whether the original data was sensible — a wrong value on the form is still wrong.
核實不會檢查原始數據是否合理——表格上本身錯誤的數值仍然是錯的。

Parity checking 奇偶校驗

A parity bit is an extra bit added to a group of bits so errors in transmission can be detected.
奇偶位元是加在一組位元上的額外位元，用以偵測傳送中的錯誤。
Even parity: the total number of 1s (including the parity bit) must be even. Odd parity: the total must be odd.
偶校驗：1 的總數（包括奇偶位元）必須是雙數。奇校驗：總數必須是單數。
Example: data 1011001 has four 1s. Even parity bit is 0; odd parity bit is 1.
例子：數據 1011001 有四個 1。偶校驗位元＝0；奇校驗位元＝1。
Limitation: if two bits change, the parity is still correct, so the error is not detected; parity also cannot show which bit is wrong.
限制：若兩個位元同時改變，奇偶性仍然正確，錯誤便不能被偵測；奇偶校驗亦不能指出哪個位元出錯。

スライド

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練習問題

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1.Arrange the levels of the data hierarchy from SMALLEST to LARGEST. 把數據層級由最小至最大排列。
Easy
- Character 字元
- Field 欄位
- Record 記錄
- File 檔案
- Database 數據庫
2.In a school's student file, the class of one student (e.g. 4C) is a ……. 在學校的學生檔案中，一名學生的班別（例如 4C）是一個……。
Easy
- AField 欄位
- BRecord 記錄
- CFile 檔案
- DDatabase 數據庫
3.A collection of related records, such as all the books in a library, is called a ……. 一組相關記錄，例如圖書館所有書籍，稱為……。
Easy
- ACharacter 字元
- BFile 檔案
- CField 欄位
- DKey field 關鍵欄位
4.Which of the following is the MOST suitable key field for a file of mobile phone customers? 以下哪項最適合作為手提電話客戶檔案的關鍵欄位？
Easy
- ACustomer name 客戶姓名
- BDate of joining 登記日期
- CCustomer account number 客戶戶口號碼
- DDistrict 地區
5.Two records in the same file may have the same value in the key field. 同一檔案中的兩個記錄可以有相同的關鍵欄位值。
Easy
True or false?
6.Which storage medium supports ONLY sequential access? 哪種儲存媒體只支援順序存取？
Easy
- AUSB flash drive USB 快閃記憶體
- BHard disk 硬碟
- CSolid-state drive 固態硬碟
- DMagnetic tape 磁帶
7.Which of the following applications requires direct access? 以下哪個應用需要直接存取？
Easy
- AChecking an Octopus card balance at a machine 在機器查詢八達通餘額
- BWeekly backup of a server 伺服器的每週備份
- CMonthly payroll for all staff 全體員工的每月薪金
- DPrinting all students' report cards 列印全體學生的成績表
8.Which of the following are advantages of direct access over sequential access? (Select all that apply) 以下哪些是直接存取相對順序存取的優點？（選出所有正確答案）
Medium
- AAny record can be retrieved quickly 可快速讀取任何記錄
- BA single record is easy to update 容易更新單一記錄
- CThe storage medium is always cheaper 儲存媒體必定較便宜
- DRecords can be inserted without rewriting the whole file 插入記錄時不需重寫整個檔案

もっと問題

9.A file of 10 000 records is stored sequentially. On average, about how many records must be read to find one record? 一個有 10 000 個記錄的檔案以順序方式儲存。平均要讀取約多少個記錄才能找到一個記錄？
Medium
- A1
- B5 000
- C100
- D10 000
10.Sequential access is efficient when most of the records in a file are processed each time. 當每次都處理檔案中大部分記錄時，順序存取很有效率。
Easy
True or false?
11.What does "garbage in, garbage out" mean? 「垃圾入，垃圾出」是甚麼意思？
Easy
- AOld computers should be recycled 舊電腦應該回收
- BDeleted files go to the recycle bin 刪除的檔案會放進資源回收筒
- CIf input data is wrong, the output will be wrong 若輸入數據錯誤，輸出亦會錯誤
- DPrograms always correct bad data 程式總會更正錯誤數據
12.A clerk types the product code 3847 as 3487. What type of error is this? 文員把貨品編號 3847 輸入成 3487。這是哪類錯誤？
Easy
- ATransmission error 傳送錯誤
- BOverflow error 溢出錯誤
- CRange error 範圍錯誤
- DTransposition error 換位錯誤
13.Which of the following BEST describes validation? 以下哪項最能描述有效性檢驗？
Easy
- AThe computer checks that data is reasonable and follows set rules 電腦檢查數據是否合理並符合設定規則
- BA person proofreads data against the source document 由人把數據與原始文件校對
- CData is typed twice and compared 數據輸入兩次並比較
- DData is encrypted before it is sent 數據在傳送前被加密
14.Match each validation check with an example. 把每種有效性檢驗配對一個例子。
Medium
- Range check 範圍檢查
- Type check 類型檢查
- Presence check 存在檢查
- Format check 格式檢查
- Month must be from 1 to 12 月份必須是 1 至 12
- Age must contain only digits 年齡只可包含數字
- The email field cannot be empty 電郵欄位不可留空
- Date must be written as DD/MM/YYYY 日期必須寫成 DD/MM/YYYY
15.The digit in brackets on a Hong Kong Identity Card, e.g. A123456(3), is used as a ……. 香港身份證上括號內的數字，例如 A123456(3)，用作……。
Medium
- AParity bit 奇偶位元
- BCheck digit 核對數位
- CKey field 關鍵欄位
- DPassword 密碼
16.A teacher enters a mark of 67 as 76. Which of the following would MOST likely detect this error? 老師把分數 67 輸入成 76。以下哪項最有可能偵測到這個錯誤？
Medium
- ARange check (0–100) 範圍檢查（0–100）
- BType check 類型檢查
- CVisual check against the mark sheet 與分數表目視核對
- DPresence check 存在檢查
17.If data passes all validation checks, it must be correct. 若數據通過所有有效性檢驗，它必定正確。
Easy
True or false?
18.When creating an account, a user must type the password twice. This is an example of ……. 開設帳戶時，使用者須把密碼輸入兩次。這是……的例子。
Easy
- Aa length check 長度檢查
- Ba range check 範圍檢查
- Ca parity check 奇偶校驗
- Ddouble entry verification 雙重輸入核實
19.Which of the following statements about verification are correct? (1) It checks that data is copied accurately from the source. (2) It can be done by double entry. (3) It can detect that the source document itself contains wrong data. 以下哪些關於核實的陳述是正確的？(1) 它檢查數據是否從來源準確地抄錄。(2) 它可以透過雙重輸入進行。(3) 它能偵測原始文件本身的錯誤數據。
Medium
- A(1) only 只有 (1)
- B(1) and (2) only 只有 (1) 和 (2)
- C(2) and (3) only 只有 (2) 和 (3)
- D(1), (2) and (3) (1)、(2) 和 (3)
20.Using ODD parity, which of the following bytes has been received WITHOUT a detectable error? 使用奇校驗，以下哪個位元組在接收時沒有可偵測的錯誤？
Medium
- A10101011
- B11000011
- C10110100
- D11110000
21.Using EVEN parity, what should the parity bit be for the 7-bit data 0110101? 使用偶校驗，7 位元數據 0110101 的奇偶位元應是甚麼？
Medium
- A1
- B0
- C2
- DIt cannot be found 無法求得
22.A parity check can always detect an error when two bits in a byte are changed during transmission. 當位元組中有兩個位元在傳送時改變，奇偶校驗必定能偵測到錯誤。
Medium
True or false?
23.Parity checking is mainly used to detect errors that occur during ……. 奇偶校驗主要用來偵測在……中發生的錯誤。
Easy
- Acopying from a paper form 從紙本表格抄錄
- Bprogram design 程式設計
- Cdata transmission 數據傳送
- Dchoosing a key field 選擇關鍵欄位
24.Which of the following are validation checks? (Select all that apply) 以下哪些是有效性檢驗？（選出所有正確答案）
Medium
- ALength check 長度檢查
- BDouble entry 雙重輸入
- CCheck digit 核對數位
- DPresence check 存在檢查
- EProofreading 校對
25.Which check makes sure Student ID is one letter S followed by five digits? 哪種檢查確保學生編號是字母 S 後接五位數字？
Medium
- ARange check 範圍檢查
- BDouble entry 雙重輸入
- CPresence check 存在檢查
- DFormat check 格式檢查
26.Which check is MOST suitable for Age? 哪種檢查最適合用於年齡？
Medium
- ARange check, e.g. 11 to 19 範圍檢查，例如 11 至 19
- BFormat check DD/MM/YYYY 格式檢查 DD/MM/YYYY
- CCheck digit 核對數位
- DLength check of 8 characters 8 個字元的長度檢查
27.The Event field uses a drop-down list with three choices. What is the main benefit? 項目欄位使用有三個選擇的下拉式清單。主要好處是甚麼？
Medium
- AIt encrypts the data 它把數據加密
- BOnly valid values can be entered 只可輸入有效值
- CIt removes the need for a key field 它令關鍵欄位變得不必要
- DIt speeds up the network 它加快網絡速度
28.Asking students to type their email twice is a form of verification. 要求學生輸入電郵兩次是一種核實。
Easy
True or false?
29.Which of the following applications need DIRECT access? (Select all that apply) 以下哪些應用需要直接存取？（選出所有正確答案）
Medium
- AAirline seat booking 機票訂位
- BMonthly gas bills for all customers 所有客戶的每月煤氣費單
- CLibrary catalogue search 圖書館目錄搜尋
- DWeekly full backup 每週完整備份
30.Why is a sequential file usually sorted by its key field? 為何順序檔案通常按關鍵欄位排序？
Medium
- ASo that it can be accessed directly 以便可以直接存取
- BSo that it takes up less space 以減少佔用空間
- CSo that records can be processed and matched in order 以便按次序處理和配對記錄
- DSo that it does not need validation 以便不需有效性檢驗
31.How many 1s are there in the byte 11011010? 位元組 11011010 中有多少個 1？
Easy
32.Name the parity bit. 寫出奇偶位元。
Medium
Using odd parity, the parity bit for the data 1100110 is ____.
33.Name the validation check that makes sure a required field is not left blank. 說出確保必填欄位不留空的有效性檢驗名稱。
Easy
34.Which of the following is NOT a reason for controlling data? 以下哪項不是控制數據的原因？
Easy
- ATo reduce wrong output caused by wrong input 減少因錯誤輸入導致的錯誤輸出
- BTo detect errors during transmission 偵測傳送時的錯誤
- CTo keep information reliable for decision-making 保持資訊可靠以作決策
- DTo make the computer's processor run faster 令電腦處理器運行得更快

復習ノート

## Data hierarchy 數據層級 - Character → field → record → file → database. 字元 → 欄位 → 記錄 → 檔案 → 數據庫。 - **Key field**: unique for every record. 關鍵欄位：每個記錄都獨一無二。 ## Access methods 存取方式 - **Sequential**: in order; tape; payroll, backups. 順序：按次序；磁帶；薪金、備份。 - **Direct**: straight to the record; disk, SSD; ATM, booking. 直接：直接到記錄；磁碟、固態硬碟；櫃員機、訂位。 ## Data control 數據控制 - GIGO: garbage in, garbage out. 垃圾入，垃圾出。 - **Validation** (automatic, reasonable?): range, type, length, presence, format, check digit. 有效性檢驗（自動，是否合理？）：範圍、類型、長度、存在、格式、核對數位。 - **Verification** (entered exactly as source?): double entry, visual check. 核實（是否與來源一致？）：雙重輸入、目視檢查。 - **Parity**: even/odd number of 1s; misses two-bit errors. 奇偶校驗：1 的數目為雙數／單數；無法偵測兩個位元的錯誤。

Field 欄位: One item of data about a person or thing. 關於某人或某事物的一項數據。
Record 記錄: A set of related fields about one person or thing. 關於一個人或一件事物的一組相關欄位。
File 檔案: A collection of related records. 一組相關記錄。
Key field 關鍵欄位: A field whose value is unique for every record. 值對每個記錄都獨一無二的欄位。
Sequential access 順序存取: Reading records one after another from the start. 由開頭逐一讀取記錄。
Direct access 直接存取: Going straight to a record without reading the others. 不讀取其他記錄而直接前往某記錄。
Validation 有效性檢驗: Automatic check that data is reasonable and follows rules. 自動檢查數據是否合理及符合規則。
Verification 核實: Check that data was entered exactly as in the source. 檢查數據是否與來源完全一致地輸入。
Check digit 核對數位: An extra digit calculated from the other digits to detect entry errors. 由其他數字計算出的額外數字，用以偵測輸入錯誤。
Parity bit 奇偶位元: An extra bit making the number of 1s even or odd, to detect transmission errors. 令 1 的數目成雙數或單數的額外位元，用以偵測傳送錯誤。

フラッシュカード

Data hierarchy, smallest to largest? 數據層級由小至大？

Character, field, record, file, database. 字元、欄位、記錄、檔案、數據庫。

What makes a good key field? 怎樣才是良好的關鍵欄位？

Its value is unique for every record, e.g. student ID. 其值對每個記錄都獨一無二，例如學生編號。

Sequential access: medium and use? 順序存取：媒體及用途？

Magnetic tape; payroll, bills, backups. 磁帶；薪金、帳單、備份。

Direct access: medium and use? 直接存取：媒體及用途？

Hard disk / SSD; ATM, bookings, catalogues. 硬碟／固態硬碟；櫃員機、訂位、目錄。

What does GIGO mean? GIGO 是甚麼意思？

Garbage in, garbage out: wrong input gives wrong output. 垃圾入，垃圾出：錯誤輸入導致錯誤輸出。

Six validation checks? 六種有效性檢驗？

Range, type, length, presence, format, check digit. 範圍、類型、長度、存在、格式、核對數位。

Two verification methods? 兩種核實方法？

Double entry and visual check (proofreading). 雙重輸入及目視檢查（校對）。

Validation vs verification? 有效性檢驗與核實的分別？

Validation: is it reasonable? Verification: was it entered as in the source? 有效性檢驗：是否合理？核實：是否與來源一致？

Even parity bit for 1011001? 1011001 的偶校驗位元？

0 — there are already four 1s. 0——已有四個 1。

Limitation of parity checking? 奇偶校驗的限制？

Cannot detect two-bit errors or show which bit is wrong. 無法偵測兩個位元的錯誤，亦不能指出哪個位元出錯。

What is a transposition error? 甚麼是換位錯誤？

Two characters swapped, e.g. 5162 typed as 5612. 兩個字元對調，例如 5162 打成 5612。

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