Concept of Cardinalilty in Oracle As per wikipedia: Cardinality refers to uniquess of data:


  In SQL (Structured Query Language), the term 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. Thus, a column with the lowest possible
  cardinality would have the same value for every row. SQL databases use
  cardinality to help determine the optimal query plan for a given
  query.


BUT, Oracle documentation explains it as:
"The cardinality is the estimated number of rows that will be returned by each operation"

So, is this term has different meaning in SQL and in Oracle ? or  i am missing something here ?
The concept of cardinality does not change whether it is used in reference to the Oracle database or some other SQL database. Cardinality is the size of a set of some elements. From the mathematics point of view a set contains only distinct elements. When you reference values of a certain column as a set its cardinality is the number of unique values. When you reference rows returned by an operation they all comprise the result set (each has a unique row ID, to put it simply) and therefore the result set cardinality is the number of rows returned.