Data Analytics Glossary

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What is Cardinality(SQL)?

In SQL, cardinality refers to the uniqueness of data in a specific column of a table. A table would be said to have less cardinality if it has more duplicated data in a column. So, more the cardinality less the data duplication (in a column) of SQL database table. In databases, the term data cardinality is used. It is related to the performance of a query. It uncovers how many unique values are in an attribute.

How to calculate Cardinality(SQL)?

A cardinality estimation framework is used which includes base statistics that estimate the foreseen number of rows that would return from the primary table. The base statistics consists of three components:

  • Table Metadata

  • Column Density

  • Column Distribution Histogram

Histograms created either manually or automatically are used for cardinality estimation CE in SQL. Logical rewrites and constraint information can also be used by SQL server to calculate cardinality.

Types and example:

For instance, suppose we have a table with a "Gender" column which has just two potential values of "Men" and "Women". At that point, that "Gender" column would have a cardinality of 2 on the grounds that there are just two one of a kind values that might show up in that column – Men and Women. A column with zero cardinality means that column does not hold any unique value.

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