Database
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Overview
Reference
Terminology
Topics
Cardinality
In the relational model, tables can be related as any of "one-to-many", "many-to-many", "one-to-zero-or-one", etc.. This is said to be the cardinality of a given table in relation to another.
| Relationship | Example | left | right |
|---|---|---|---|
| one-to-one | person ←→ id card | 1 | 1 |
| one-to-one (optional on one side) | person ←→ driving license | 1 | 0..1 or ? |
| many-to-one | person ←→ birth place | 1..* or + | 1 |
| many-to-many (optional on both sides) | person ←→ book | 0..* or * | 0..* or * |