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2. Multiset - Wikipedia

   en.wikipedia.org/wiki/Multiset

   The cardinality or "size" of a multiset is the sum of the multiplicities of all its elements. For example, in the multiset {a, a, b, b, b, c} the multiplicities of the members a, b, and c are respectively 2, 3, and 1, and therefore the cardinality of this multiset is 6.

3. HyperLogLog - Wikipedia

   en.wikipedia.org/wiki/HyperLogLog

   HyperLogLog is an algorithm for the count-distinct problem, approximating the number of distinct elements in a multiset. [1] Calculating the exact cardinality of the distinct elements of a multiset requires an amount of memory proportional to the cardinality, which is impractical for very large data sets. Probabilistic cardinality estimators ...

4. Relational algebra - Wikipedia

   en.wikipedia.org/wiki/Relational_algebra

   The relational algebra uses set union, set difference, and Cartesian product from set theory, and adds additional constraints to these operators to create new ones.. For set union and set difference, the two relations involved must be union-compatible—that is, the two relations must have the same set of attributes.

5. Cardinality (data modeling) - Wikipedia

   en.wikipedia.org/wiki/Cardinality_(data_modeling)

   Within data modelling, cardinality is the numerical relationship between rows of one table and rows in another. Common cardinalities include one-to-one , one-to-many , and many-to-many . Cardinality can be used to define data models as well as analyze entities within datasets.

6. Cardinality (SQL statements) - Wikipedia

   en.wikipedia.org/wiki/Cardinality_(SQL_statements)

   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.

7. Set (abstract data type) - Wikipedia

   en.wikipedia.org/wiki/Set_(abstract_data_type)

   In relational databases, a table can be a (mathematical) set or a multiset, depending on the presence of unicity constraints on some columns (which turns it into a candidate key). SQL allows the selection of rows from a relational table: this operation will in general yield a multiset, unless the keyword DISTINCT is used to force the rows to be ...