What is cardinality in Databases

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I have been searching all over the internet but I couldn t seem to find an answer that I can understand   So kindly  if somebody could explain to me with the help of examples what is cardinality in databases   Thank you

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Cardinality refers to the uniqueness of data contained in a column  If a column has a lot of duplicate data  e g  a column that stores either  true  or  false    it has low cardinality  but if the values are highly unique  e g  Social Security numbers   it has high cardinality

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Cardinality of a set is the namber of the elements in set for we have a set a   a b c  lt  so ths set contain 3 elements  3 is the cardinality of that set

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It depends a bit on context   Cardinality means the number of something but it gets used in a variety of contexts    When you re building a data model  cardinality often refers to the number of rows in table A that relate to table B   That is  are there 1 row in B for every row in A  1 1   are there N rows in B for every row in A  1 N   are there M rows in B for every N rows in A  N M   etc  When you are looking at things like whether it would be more efficient to use a b -tree index or a bitmap index or how selective a predicate is  cardinality refers to the number of distinct values in a particular column   If you have a PERSON table  for example  GENDER is likely to be a very low cardinality column  there are probably only two values in GENDER  while PERSON ID is likely to be a very high cardinality column  every row will have a different value   When you are looking at query plans  cardinality refers to the number of rows that are expected to be returned from a particular operation    There are probably other situations where people talk about cardinality using a different context and mean something else

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A source of confusion may be the use of the word in two different contexts - data modelling and database query optimization   In data modelling terms  cardinality is how one table relates to another    1-1  one row in table A relates to one row in tableB  1-Many  one row in table A relates to many rows in tableB  Many-Many  Many rows in table A relate to many rows in tableB    There are also optional participation conditions to the above  where a row in one table doesn t have to relate to the other table at all    See Wikipedia on Cardinality  data modelling      When talking about database query optimization  cardinality refers to the data in a column of a table  specifically how many unique values are in it  This statistic helps with planning queries and optimizing the execution plans   See Wikipedia on Cardinality  SQL statements

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In database  Cardinality number of rows in the table     img source     img source     Relationships are named and classified by their cardinality  i e  number of elements of the set      Symbols which appears closes to the entity is Maximum cardinality and the other one is Minimum cardinality  Entity relation  shows end of the relationship line as follows       image source

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Definition  We have tables in database  In relational database  we have relations among the tables  These relations can be one-to-one  one-to-many or many-to-many  These relations are called  cardinality    Significant of cardinality   Many relational databases have been designed following stick business rules When you design the database we define the cardinality based on the business rules  But every objects has its own nature as well   When you define cardinality among object you have to consider all these things to define the correct cardinality

[database] What is cardinality in Databases?

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