Joins

Joins are fundamental operations in relational databases that combine rows from two or more tables based on a related column, enabling the construction of richer result sets from normalized data. They rely on a join condition that specifies how rows in one table relate to rows in another, and they are a core tool for querying interconnected data.

Inner join returns rows with matching values in both tables. Left outer join returns all rows from the left table and matching rows from the right table, with nulls where there is no match. Right outer join is the mirror image, returning all rows from the right table and matching rows from the left, with nulls where needed. Full outer join returns all rows when there is a match in either table, filling in nulls for missing sides. Cross join produces the Cartesian product of the two tables, pairing every row from the first table with every row from the second. Natural join automatically joins on all columns with the same name and compatible data types. Self join is a join of a table to itself, typically using aliases to distinguish the two sides.

In SQL, joins are written with the JOIN keyword and an ON clause that specifies the condition; the USING clause can also specify common columns. Performance and result accuracy depend on correct join predicates and appropriate indexing, as poorly chosen or missing conditions can lead to excessive processing or unintended results. Aliases and explicit join clauses help readability and prevent ambiguity.

Example: SELECT a.id, b.value FROM A AS a INNER JOIN B AS b ON a.id = b.a_id;