SQL Interview Questions — High Cardinality Vs Low Cardinality

In this tutorial, you will learn "How to use High Cardinality and Low Cardinality in SQL" in SQL Server.

In SQL and database design, cardinality refers to the uniqueness of data values contained in a column. Understanding the difference between high cardinality and low cardinality can help optimize database performance and query efficiency. In fact Cardinality refers to the number of unique values in a column relative to a table's total number of rows.

💫High cardinality means that the column contains a large number of unique values. Columns with high cardinality are often those that are used to uniquely identify rows, such as IDs, email addresses, or other unique identifiers.

Characteristics of High cardinality:

👍Unique Values: Each value in the column is unique or nearly unique. 👍Examples: User IDs, email addresses, social security numbers. 👍Implications: 👍Indexing: Indexing high cardinality columns can be very effective because it allows quick lookups. 👍Performance: Queries that filter or sort by high cardinality columns can be more efficient with proper indexing. 👍Storage: Indexes on high cardinality columns might consume more space.

💫Low cardinality means that the column contains a small number of unique values. Columns with low cardinality often include categorical data with limited distinct values.

Characteristics of Low cardinality:

👍Few Unique Values: Many rows share the same value. 👍Examples: Gender, status flags, boolean values, country codes. 👍Implications: 👍Indexing: Indexing low cardinality columns might not be as beneficial since many rows have the same value. 👍Performance: While indexing might not provide significant benefits, low cardinality columns can be useful in GROUP BY or PARTITION BY operations. 👍Storage: Indexes on low cardinality columns are generally smaller.

🧑🏻‍🎓 Understanding the cardinality of columns is crucial for database design and query optimization. High cardinality columns are ideal candidates for indexing, while low cardinality columns might not benefit as much from indexing but can be useful for categorical data analysis.

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