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Author: JPHaus

Concepts/Cardinality.md

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+Cardinality has different meanings depending on the context of what's being discussed. In data engineering, we mainly use cardinality when talking about a data model's relationships and when talking about the statistics of a particular column in a database table.
+
+## Cardinality (Data Modeling)
+
+The number of rows in one table that relate to rows in another table. Common types include one-to-one, one-to-many, and many-to-many. It's used to define and analyze relationships in data models.[^1]
+
+## Cardinality (SQL Statements)
+
+The number of distinct values in a column/expression. Low cardinality means many repeated values, while high cardinality means most values are unique.[^2] Database engines can use this information to optimize SQL queries.
+
+[^1]: https://en.wikipedia.org/wiki/Cardinality_%28data_modeling%29
+[^2]: https://en.wikipedia.org/wiki/Cardinality_%28SQL_statements%29
+
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