What Is Cardinality in Data Structure?


Larry Thompson

Cardinality is a fundamental concept in data structure that refers to the number of elements in a set or a sequence. It helps us understand the size or the scale of the data structure. In this article, we will explore cardinality in detail and its significance in data analysis and database management.

Understanding Cardinality

Cardinality provides insights into how many unique values are present in a set or a sequence. It quantifies the uniqueness of data and plays a crucial role in various aspects of data analysis, such as query optimization, index design, and database performance tuning.

In the context of databases, cardinality is commonly used to describe relationships between tables. It defines how many distinct records from one table are associated with each record in another table.

Types of Cardinality

There are three main types of cardinality:

  • Low Cardinality: Low cardinality indicates that there are few unique values present in a dataset. For example, if we have an attribute called “Gender” with only two possible values – “Male” and “Female”, it has low cardinality.
  • High Cardinality: High cardinality means that there are many unique values present in a dataset.

    For instance, if we have an attribute called “Email Address” where each entry is unique, it has high cardinality.

  • Unique Cardinality: Unique cardinality refers to a situation where each value within a dataset is unique. This implies that every element has its own distinct value.

The Importance of Cardinality

The understanding of cardinality is crucial for optimizing database performance. It helps database administrators make informed decisions when designing indexes, executing queries, and improving overall system efficiency.

By analyzing cardinality, one can identify attributes that are suitable for indexing. High cardinality attributes tend to be good candidates for indexing as they offer greater selectivity, allowing the database system to fetch relevant data more efficiently. On the other hand, low cardinality attributes may not provide significant benefits when indexed.

Furthermore, cardinality plays a vital role in query optimization. The database optimizer utilizes cardinality estimation to determine the most efficient execution plan for a query. Accurate estimations of cardinality help in minimizing resource consumption and query execution time.


In summary, cardinality is an essential concept in data structure that quantifies the uniqueness of elements within a set or a sequence. It aids in optimizing database performance, index design, and query execution. By understanding the different types of cardinality and their implications, database professionals can make informed decisions regarding data analysis and management.

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