The big idea: A relational database stores data in tables, and links those tables together.
Each fact is stored once, in the table it belongs to, and other tables point at it rather than copying it.
Why not one big table: Put students and their clubs in one table and a teacher's room gets written out on every row.
Change the room and you must find every copy. Miss one and the database now disagrees with itself.
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Tables and relationships
- A table holds one kind of thing: students, clubs, teachers
- Each row is one of those things; each column is one property
- A relationship links a row in one table to rows in another
Keys that identify
- Primary key — uniquely identifies each row in its table
- Composite key — two or more columns used together as the primary key
- Needed when no single column is unique on its own
Keys that link
- Foreign key — a column holding a primary key from another table
- That is how the link is physically made
- The database refuses a foreign key pointing at a row that does not exist
Primary against foreign, in one line: A primary key says which row this is. A foreign key says which row in the other table this belongs to.
One identifies; the other points.
How relationships are drawn, and how each is built with keys. Step through all four.
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Benefits
- Reduced duplication — each fact stored once, so less storage and fewer contradictions
- Data integrity — the database enforces the rules, such as refusing an invalid foreign key
- Consistency — change a fact in one place and everyone sees the change
- Concurrency control — many people can work at once without corrupting each other's changes
- Security — access can be granted per table or per column
Limitations
- Rigid schema — the structure must be decided up front and is awkward to change
- Design complexity — getting the tables right takes real skill
- Unstructured data — documents, images and free text fit badly into rows and columns
- Hierarchical data — deeply nested structures need awkward self-joins
- Scalability — very large data is hard to spread across many machines
Why the rigid schema is the usual complaint: Adding a column to a table with millions of rows is slow and disruptive, and every program reading that table may need changing.
That inflexibility is the price of the guarantees on the other side.
How this is tested — you must name a feature or benefit AND say what it prevents or enables. It comes up two ways:
Paper 1 Section A
- Explain features, benefits or limitations, 3-5 marks
- Distinguish primary, foreign and composite keys
- Say why a relational database suits a scenario
Paper 1 Section B — case study
- Evaluate the case study's data storage
- Say whether relational is the right choice for them
The classic trap: Listing benefits as single words — "integrity, consistency, security". Each needs a clause saying what it prevents: an invalid foreign key refused, one update seen by everyone.
A school currently keeps student and club information in one large spreadsheet. Explain two benefits of moving to a relational database.
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