About
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.
You can:
- answer practical question such as which products in the Product table have the same names but different types of packaging?
- or build a hierarchy
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Example
| table2 (inner and outer set) |
|
|---|---|
| Column ID | Column ID_2 |
| A | |
| B | A |
| C | C |
gerardnico@orcl>select t1.id "ID_table2"
2 from table2 t1, table2 t2
3 where t1.id = t2.id_2;
ID_table2
----------
A
C