A join does not always have to involve two different tables. You can join a table to itself, creating a self-join. Joining a table to itself can be useful when you want to compare values in a column to other values in the same column.
- answer practical question such as which products in the Product table have the same names but different types of packaging?
- or build a hierarchy
(inner and outer set)
|Column ID||Column ID_2|
[email protected]>select t1.id "ID_table2" 2 from table2 t1, table2 t2 3 where t1.id = t2.id_2; ID_table2 ---------- A C