Joins within a table

Hello all -

I'm trying to do a JOIN within a table. In MySQL, I would do

SELECT  main_table.field, join_table.field
FROM main_table
LEFT JOIN main_table AS join_table ON join_table.id = main_table.parent_id

Or something like that.

However, in the Postgres documentation, I don't see anything on JOIN AS.
How would I do a join within a table in Postgres?

The table in question is a genetics table of viruses. The columns are
id, strain, and parent_id. Since viruses only replicate, each strain has
one parent. This way I can have one table and show entire tree of the
relationship. It guarantees that I have only one parent per virus.

The only other way I can think of doing it would be to have two tables:

table strains
------------
id
strain

table relationships
--------------
parent_id
child_id


But then I think I would have to implement some constraints so that I
don't get more than one parent per child.