The big idea: A relational database stores data in tables with a fixed schema, and that is the right choice far more often than not.
When other models fit: But when the data has no fixed shape, or must be spread across hundreds of machines, other models trade some of relational's guarantees for what those situations need.
| Type | Stores data as | Suits |
|---|---|---|
| Relational | Tables, fixed schema, joins | Structured data with relationships and transactions |
| Document | Self-contained documents (JSON-like) | Records whose fields vary between rows |
| Key-value | A key and an opaque value | Caches, sessions — lookup by one key, very fast |
| Graph | Nodes and the edges between them | Relationships that are themselves the data |
| Column-family | Columns grouped, not rows | Huge tables where queries read few columns |
The four models the guide names
| Model | What it is | Real-world use |
|---|---|---|
| NoSQL | Any non-relational store: document, key-value, graph or column-family (above) | Social media posts and friend links; e-commerce product catalogues |
| Cloud | A database run by a provider as a managed service, scaled and backed up for you, paid for by use | A SaaS app that grows from 100 to 100,000 users without buying servers |
| Spatial | Stores locations and shapes (points, lines, areas) and answers questions such as "what lies within 2 km?" | Geographic information systems (GIS): maps, delivery routes, tracking an outbreak by address |
| In-memory | Keeps the data in RAM instead of on disk, so reads and writes are very fast; copies to disk are needed for durability | Real-time analytics: live dashboards, fraud checks during a payment |
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Document
- No fixed schema — two records may hold different fields
- A whole record is read in one go, with no joins
- But duplicated data must be updated everywhere it appears
Key-value
- The simplest possible model: give a key, get a value
- Extremely fast, and easy to spread across machines
- You cannot query the value — only fetch it
Graph
- Nodes joined by edges that carry meaning
- "Friends of friends who like X" is one traversal
- In SQL that is a join per level, and it degrades fast
The relational cost that drives this: A relational query answering "friends of friends of friends" needs three joins, and each one multiplies the work. A graph database follows edges directly, so depth costs far less.
Where the relationships are the question, the relational model is doing the wrong kind of work.
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The honest trade
- No enforced schema means nothing stops a record missing a field it needs
- No joins means duplication, and duplication means update anomalies return
- Weaker transactions — many non-relational stores offer only eventual consistency
- Fewer guarantees overall: the database no longer enforces what you assumed
- And SQL skills do not transfer — every system has its own query language
"NoSQL is more modern" is not a justification: These are trades, not upgrades. Relational databases remain the right answer for most business data precisely because they enforce structure and transactions.
An exam answer choosing a document store must say what about this data does not fit tables.
How this is tested — you must match a storage model to what the data and the queries actually look like. It comes up two ways:
Paper 1 Section A
- Outline the different types of database, 3-4 marks
- Identify a suitable type for a scenario
- State one feature of a graph or document store
Paper 1 Section B — case study
- Justify a choice against the relational default
- Explain what the choice gives up
The classic trap: Choosing a non-relational store because it sounds modern. These are trades: an answer must say what about this data does not fit tables, and what guarantee is being surrendered in exchange.
IB-style questionSuggest[5 marks]
A social network must answer "which of my friends' friends live in my city?" for millions of users. Suggest a suitable database type, justify it, and state one drawback.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.