# Data Modeling - Cardinality

> (Data|State) Management and Processing > Data Modeling (RDBMS or Code)

### Table of Contents

## 1 - About

The cardinality is way to define the relationship between two entity 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 entity 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

## 2 - Articles Related

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## 3 - The cardinality between two entities 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

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## 4 - 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 |