Okuzeka
Databases & SQL vocabulary · Data Modeling and Normalization

Cardinality

Meaning in databases & SQL, with examples

/ˌkɑːrdɪˈnæləti/nounAdvanced term

In databases & SQL, cardinality means: Either the type of relationship between tables (one-to-one, one-to-many, many-to-many) or the number of distinct values in a column.

In Turkish: kardinalite, eleman sayısı

How it is used

Context decides the meaning. In data modeling it describes relationships on an ER diagram. In performance work, 'high cardinality' means many distinct values (like email) and 'low cardinality' means few (like status); high-cardinality columns usually make better index candidates.

Example sentences

  1. An index on the gender column won't help much because its cardinality is very low.

  2. Check the cardinality of each relationship before you draw the foreign keys.

Common collocations

  • high cardinality
  • low cardinality
  • relationship cardinality
  • cardinality estimate
  • one-to-many relationshipA link between two tables where one row in the first table can be connected to many rows in the second, but each of those belongs to only one, like a customer and their orders.
  • indexAn extra data structure built on one or more columns that lets the database find rows quickly without reading the whole table, like the index at the back of a book.
  • query optimizerThe part of a database engine that looks at a query and the table statistics and picks the cheapest way to run it, producing the execution plan.
  • ER diagramA drawing of a database design that shows the entities (usually tables), their attributes and the relationships between them, such as one-to-many.

This word in other fields

The same English word can have a different meaning in another field:

More terms: Data Modeling and Normalization

Practise with graded reading

Reading at the right level is the fastest way to make new words stick. Pick free English books by level: