OUTER JOIN (Relational Operation)

In NATURAL JOIN (Relational Operation) and EQUIJOIN (Relational Operation), tuples without a matching tuple are eliminated from the join result.

• Tuples with null in the join attributes are also eliminated. We can use outer joins to conditionally keep certain data we may want.

• LEFT OUTER JOIN: $R\text{ ⟕}S$
  Keeps every tuple in the left relation $R$. If no matching tuple is found in $S$, then the attributes of $S$ in the join result are filled with null values.

• RIGHT OUTER JOIN: $R\text{ ⟖}S$
  Keeps every tuple in the right relation $S$. Attributes of $R$ are filled with null values in unlatching tuples.

• FULL OUTER JOIN: $R\text{ ⟗}S$
  Keeps every tuple in both relations. When no matching tuples are found, pad with null values.