About
The cardinality is way to define the relationship between two relation in a data model :
- optional on one side one-to-one
- …
Cardinalities further describe a join between 2 entity by stating how many rows in one relation will match rows in an another (It defines the minimum and maximum number of occurrences of one entity for a single occurrence of the related entity).
Cardinality is not a number; it is a range
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The cardinality between two tables define the relationship
The cardinality of one data table known as :
- left table
- inner table
- of local table
with respect to another data table known as :
- right table
- outer table
- of remote table
define the type of relationships.
| Left table cardinality | Right table cardinality | Type of relationship | ||
|---|---|---|---|---|
| Min | Max | Min | Max | |
| 1 | 1 | 1 | 1 | one-to-one |
| 0 | 1 | 1 | 1 | optional on one side one-to-one |
| 0 | n (or *) | 0 | n (or *) | optional on both sides many-to-many |
| 1 | 1 | 1 | n (or *) | one-to-many |
- Min = Minimum count of row
- Max = Maximum count of row
Cardinality Symbol
| Cardinality | Min Instance | Max Instance | Graphic Notation |
|---|---|---|---|
| Exactly one | 1 | 1 |  |
| Zero or one | 0 | 1 |  |
| One or more | 1 | many (>1) |  |
| Zero, one, or more | 0 | many (>1) |  |
| More than one | >1 | >1 |  |
| Cardinality | symbol Example | Description |
|---|---|---|
| Arrow |  | Arrow indicates the “one” direction of the join. If cardinality is 1,1 then an arrow head is shown at each join end. |
| Parity |  | Crow's foot indicates the “many” end of the join. If cardinality is 1,1, then a straight line is shown. |
| 1,N |  | Cardinality symbol Example Description Cardinality is shown as a ratio at each end of the join. |
Visualization
