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Roby Cigar
Roby Cigar

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Cardinality In Database

A source of confusion may be the use of the word in two different contexts - data modelling and database query optimization.

In data modelling terms, cardinality is how one table relates to another.

1-1 (one row in table A relates to one row in tableB)
1-Many (one row in table A relates to many rows in tableB)
Many-Many (Many rows in table A relate to many rows in tableB)

There are also optional participation conditions to the above (where a row in one table doesn't have to relate to the other table at all).

When talking about database query optimization, cardinality refers to the data in a column of a table, specifically how many unique values are in it. This statistic helps with planning queries and optimizing the execution plans.

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