title: What is the definition of cardinality in SQL
body: <p>My school book <a href="https://rads.stackoverflow.com/amzn/click/com/0321523067" rel="noreferrer" rel="nofollow noreferrer">Database Systems</a> defines cardinality as follows:</p>
  
  <blockquote>
    <p>The cardinality of a relation is the number of tuples it contains. By contrast, the number of tuples is called the cardinality of the relation and this changes as tuples are added or deleted. High-cardinality - many tuples, low-cardinality - few tuples.</p>
  </blockquote>
  
  <p>While the Wikipedia article on <a href="https://en.wikipedia.org/wiki/Cardinality_(SQL_statements)" rel="noreferrer">Cardinality (SQL statements)</a>, defines it as follows:</p>
  
  <blockquote>
    <p>Cardinality refers to the uniqueness of data values contained in a particular column (attribute) of a database table. The lower the cardinality, the more duplicated elements in a column. There are 3 types of cardinality: high-cardinality, normal-cardinality, and low-cardinality.</p>
  </blockquote>
  
  <p>They might both be right, but i can't connect the two definitions as related definitions. A rephrase would be appriciated!</p>