distinctoperation

Distinctoperation is a generic data-processing operation that, given a collection, returns a new collection containing only unique elements, removing duplicates. It is commonly implemented as transforming a multiset into a set by discarding repeated items. The exact behavior can depend on the language or framework, particularly regarding whether element order is preserved.

In relational algebra and SQL, the distinct concept is used to produce a relation containing only distinct tuples. SQL implements this as SELECT DISTINCT to eliminate duplicate rows from query results. In relational algebra, Distinct yields a relation with no duplicate tuples, independent of the source of the data.

In programming languages, distinct is often a higher-order function or method that takes a sequence and an optional key function. It returns a sequence of elements that are unique by the key. Implementations may preserve input order (stable distinct) or not, depending on the language and library design. Custom comparers or key selectors enable defining what counts as a duplicate.

Complexity and resource use depend on the approach. Using a hash-based set typically achieves O(n) time with O(k) extra space, where k is the number of distinct elements. For streaming data or very large datasets, exact deduplication may be impractical without buffering; approximate methods and probabilistic data structures, such as HyperLogLog or Bloom filters, can estimate distinct counts with reduced memory.

Handling of equality is central to distinct operation. The notion of duplicates is determined by an equality relation or a user-supplied key; this allows tailoring deduplication to domain-specific criteria.