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Next Oct 16 Previous

The cardinal rules of SQL

When working with SQL, especially when optimizing it, you can come across the term 'high-cardinality column'. What does it mean?

A: The column data is referenced by many other tables.
B: The column data references many other tables.
C: It is a measure for the uniqueness of the data in the column.
D: The column fields can contain much data (f.i. BLOB or text fields)


Cardinality says something about the uniqueness of data in a database column. A column that contains user ids will be a (very) high-cardinality column, while the column that contains the user's sex will be a very low-cardinality column. The higher the cardinality of a column the more efficient indexed searches will be. So answer C is correct.

Whether or not a column or table is referenced by many other fields is not an important database statistic. And if data from a single column references many other tables is time to re-engineer the entire application. So answers A and B are false.

Fields that can contain much data are likely to be more unique than fields that just can contain one letter. In this way answer D has something to do with cardinality, but as an answer to the question it is false.