Thread: 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.

--- Steve Lefevre <lefevre.10@osu.edu> wrote:

> 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

PostgreSQL conforms to the SQL standard when it comes to giving alias names to a single table and
doesn't use MySQL's vendor specific extension:

         SELECT Instance1.field, Instance2.field
           FROM Main_table AS Instance1
LEFT OUTER JOIN Main_table AS Instance2
             ON Instance1.id = Instance2.id;

Here I am showing two alias names, but you could reduce this query to use only one if you wanted.

Regards,
Richard Broersma Jr.

--

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Andrew J. P. Maclean
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Steve Lefevre <lefevre.10@osu.edu> writes:
> 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

Yeah, that's the same way you'd do a self-join in Postgres.

> However, in the Postgres documentation, I don't see anything on JOIN AS.

Where did you look?  Maybe you are confusing the alias clause with the
join clause --- they are two different constructs.

            regards, tom lane