joinoperasjon

Joinoperasjon, or join operation, is a fundamental concept in relational databases and relational algebra. It combines rows from two or more tables based on a related column or condition, creating a result set that presents related data together. In SQL and many query languages, the join is expressed with the JOIN keyword and a join condition.

Common join types include inner join, outer joins (left, right, and full), and cross join. An inner join returns only rows with matching values in both tables. A left outer join returns all rows from the left table and matching rows from the right, with nulls where there is no match. Right outer join is analogous on the right, and full outer join returns all rows when there is a match or not. A cross join yields the Cartesian product of the two tables.

Join conditions typically equate columns (equality joins) but can be more general (theta-joins). Example: SELECT e.name, d.name FROM employees e JOIN departments d ON e.dept_id = d.id. This retrieves each employee with their department. A left join can include employees without a department.

Implementation and performance depend on the database system and data distribution. The query optimizer may choose algorithms such as nested-loop, hash join, or sort-merge join, and may use indexes on join columns to improve speed.

Joinoperasjon is central to relational data modeling, enabling normalization and flexible data retrieval across related tables. See also relational databases, SQL, and relational algebra.