Equijoins

An equijoin is a type of join in relational databases in which the join condition asserts equality between one or more attributes of the two relations being joined. It is a specific case of theta joins, which cover arbitrary comparison operators.

In SQL, equijoins are most commonly expressed as inner joins with an equality predicate. For example: SELECT ... FROM Orders o INNER JOIN Customers c ON o.CustomerID = c.CustomerID. The result contains only rows where the join column values match. A natural join is a related construct that automatically matches on all columns with the same name and is effectively an equijoin on those columns.

Equijoins can be implemented by different join algorithms, including hash join, sort-merge join, and nested-loop join. Indexes on the join columns can greatly improve performance by accelerating lookups and reducing the amount of data processed in the join.

Note that while the term equijoin is often used to describe inner joins, equijoins can also be formed in the context of outer joins if the join condition uses an equality predicate. In that case, non-matching rows are retained with NULLs in the side that does not match.

See also: theta join, natural join, outer join, cross join, join.