Keys & ER Model
Keys uniquely identify rows and link tables. The ER model describes entities, attributes and relationships before building tables.
Cheat sheet
- ▸Super key: any set that uniquely identifies a row.
- ▸Candidate key: minimal super key. Primary key: chosen candidate key (NOT NULL).
- ▸Alternate key: candidate keys not chosen as primary.
- ▸Foreign key: references a primary key in another table.
- ▸Composite key: key with multiple columns. Surrogate key: system-generated ID.
- ▸Cardinality: 1:1, 1:N, M:N (M:N needs a junction table).
Primary vs Unique vs Foreign
| Key | NULL allowed? | Per table | Purpose |
|---|---|---|---|
| Primary | No | One | Row identity |
| Unique | Yes (usually one/many) | Many | No duplicates |
| Foreign | Yes | Many | Referential integrity |
Top 10 campus interview questions
Quick quiz
1. A minimal super key is a:
2. Primary key can contain NULL?
3. M:N relationship is implemented using:
4. Weak entity is shown in ER with:
5. Foreign key ensures: