2.1 · Hierarchy of Data

Goal: name the five levels of the data hierarchy and identify each one in a given scenario.

The five levels

Data is organised hierarchically — from the tiniest 0/1 up to a full database. Memorise this ladder:

                       ┌─────────────────────┐
                       │     Database        │  collection of related files
                       └────────▲────────────┘
                                │
                       ┌─────────────────────┐
                       │     File            │  collection of related records
                       └────────▲────────────┘
                                │
                       ┌─────────────────────┐
                       │     Record          │  collection of related fields
                       └────────▲────────────┘
                                │
                       ┌─────────────────────┐
                       │     Field           │  the smallest meaningful unit
                       └────────▲────────────┘
                                │
                       ┌─────────────────────┐
                       │     Byte (8 bits)   │  storage of a character
                       └────────▲────────────┘
                                │
                       ┌─────────────────────┐
                       │     Bit             │  0 or 1
                       └─────────────────────┘

For Paper 1 the 5 levels you must name are:

1. Field
2. Record
3. File
4. Database

(plus the lower-level technical building blocks bit and byte)

The HKEAA usually asks: "Identify data, records, fields, files and databases in this scenario." So we focus on the top 4 + the conceptual base "data".

Walk-through · School student database

Level	Example
Database	The school's entire school_system database
File / Table	The students table
Record	One row: 1001, Alice Chan, F.4A, 2007-05-12, 86 Robinson Rd
Field	class = 'F.4A' (one cell)
Byte / Character	The letter F in F.4A
Bit	A single 0 or 1 inside the byte that stores F

More vocabulary you should know

Term	Meaning
Entity	The "thing" you're recording (a student, a book, an order)
Attribute	A characteristic of an entity (matches "field")
Tuple / Row	A specific instance of an entity (matches "record")
Relation / Table	A structured collection (matches "file" in relational DBs)
Schema	The blueprint of a database (which tables, which fields)

These appear again in Elective 2A. Knowing them now saves time later.

Real-life examples

Domain	Field	Record	File	Database
Hospital	patient_id	One patient's appointment	All appointments today	Hospital management system
Library	ISBN	One loan transaction	All loans this month	Library DB
MTR	card_id	One tap	All taps today	Octopus DB
Shop	sku	One product line on a receipt	All receipts this hour	POS DB

How data is organised inside a file

The file you store on disk can be laid out in several ways:

Layout	Description	Typical use
Sequential	Records stored one after another in a fixed order (often sorted by key)	Tape backups, log files
Indexed sequential (ISAM)	Sequential + an index allowing quick jumps	Older mainframe systems
Random / direct (hash)	Records placed at positions calculated from the key	Hash-based databases
Relational	Tables with rows and columns; relationships expressed by foreign keys	MySQL, PostgreSQL, SQLite

For Paper 1 you only need to distinguish sequential and direct access (next sub-topic).

Common student mistakes

• Confusing record and field: a record is the whole row; a field is one cell.
• Calling the whole spreadsheet a record.
• Saying byte ≠ character: in basic ASCII a character is 1 byte, but in Unicode (UTF-8) it can be 1–4 bytes.
• Forgetting that a database can contain many files/tables — not just one.

Practice activity

You are given the following CSV file from a sports club:

member_id,name,join_date,birth_year,sport
1001,Alice Chan,2024-09-01,2007,Basketball
1002,Bob Wong,2024-09-15,2008,Swimming
1003,Carol Yip,2024-10-03,2007,Tennis

For this file, identify:

• A field (e.g. member_id or any single cell value)
• A record (one full row, e.g. row of member 1002)
• The file (the CSV itself)
• A database (the club's collection of CSVs / tables)
• A byte (the storage of one character in the CSV)

Exam-style question

Q (4 marks): A car park system records the licence plate, entry time, exit time and fee for each parking event. Identify, with one example each, the field, record, file and database used by the system.

Mark scheme expects:

• Field (1): e.g. licence_plate = "AB1234"
• Record (1): one parking event row, e.g. ("AB1234", "2026-06-24 09:00", "2026-06-24 11:30", 60)
• File (1): all parking events for one day (or one location)
• Database (1): the car park company's full database covering all car parks and dates

Key takeaways

• Field → Record → File → Database.
• A field is one piece of data, a record is one instance of an entity.
• The terms reappear as attribute, tuple, table, schema in Elective 2A.

➡️ Next: 2.2 Sequential vs Direct Access