The big idea: An ERD shows the things a database stores and how they relate.
Each box is an entity — one kind of thing, which will become a table. Each line is a relationship between two of them.
Why draw it at all: A mistake in an ERD costs a pencil line. The same mistake found after the database is full of data costs a rebuild.
It is also the one document a non-technical person can check and correct.
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Each relationship type, read both ways, with how it is built in tables. Step through all four.
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Cardinality — how many
- One to one — each side has at most one of the other
- One to many — one on one side, several on the other
- Many to many — several on both sides
- Drawn with a bar for one and a crow's foot for many
Modality — is it compulsory
- Mandatory — there must be at least one
- Optional — there may be none
- Drawn with a second bar for mandatory, a circle for optional
- A student may have no club yet: optional on the club side
Read every relationship twice: Once each way. "A teacher teaches many classes" and "a class is taught by one teacher".
Both sentences together give you the cardinality. Get into the habit and you will never draw the crow's foot on the wrong end.
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1 — Find the entities
Read the scenario and pick out the things it keeps data about — usually nouns that appear repeatedly.
A library scenario gives Book, Member, Loan. Each becomes a box.
2 — Find the relationships
Look for the verbs joining those nouns: a member borrows a book.
Draw one line per relationship and label it with that verb.
3 — Decide cardinality both ways
Say the sentence in each direction. "A member borrows many books"; "a book is borrowed by many members, over time".
Many to many — which tells you a linking table is coming.
4 — Resolve any many-to-many
A relational database cannot store many-to-many directly, so replace it with a linking entity between the two.
Loan sits between Member and Book, and becomes one to many on both sides.
The linking entity usually has a name: It is rarely just glue. Member-to-Book becomes Loan, which also holds the date borrowed and the date due.
If you cannot think of a name for it, you have probably missed data that belongs on it.
How this is tested — you must draw the right cardinality on the right end, and resolve any many-to-many. It comes up two ways:
Paper 1 Section A
- Construct an ERD from a scenario, 4-6 marks
- State the cardinality of a described relationship
- Explain what a linking table is for
Paper 1 Section B — case study
- Draw the ERD for the case study's data
- Explain why a particular relationship takes the form it does
The classic trap: Leaving a many-to-many undrawn. A relational database cannot store it — there is nowhere to put a foreign key holding several values — so the linking entity is part of the answer, not an optional extra.
A hospital records which doctors treat which patients, and each patient is assigned one ward. Construct an ERD for this, explaining your cardinalities.
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