PostgreSQL Development Essentials

Using the Self join

The tables we are joining don't have to be different ones. We can join a table with itself. This is called a self join. In this case, we will use aliases for the table; otherwise, PostgreSQL will not know which column of which table instance we mean. To join a table with itself means that each row of the table is combined with itself, and with every other row of the table. The self join can be viewed as a joining of two copies of the same table. The table is not actually copied but SQL carries out the command as though it were.

The syntax of the command to join a table with itself is almost the same as that of joining two different tables:

SELECT a.column_name, b.column_name...
FROM table1 a, table1 b 
WHERE condition1 and/or ...