Gerard Samuel wrote:
Im trying to figure out Self Joins with PostgreSQL. The output of the
second SQL is correct, because of the where a.id = b.pid,
but I would like to return all rows that are part of the tree.
test=# select * from topics;
id | pid | topicname
------------------+------------------+------------
AFAdDFoAPNX6wKbr | 0 | Foo
AFAdDFoAPgTi9tAE | 0 | Apache
AFAdDFoAPgTjCa4V | AFAdDFoAPgTi9tAE | PHP
AFAdDFoAPlv1ENRn | 0 | XHTML
AFAdDFoAPoSEWZaq | 0 | News
AFAdDFoAPoSEaRPV | AFAdDFoAPoSEWZaq | World News
AFAdDFoAPoSEee5_ | AFAdDFoAPoSEWZaq | Tech News
(7 rows)
test=# select a.topicname as parent, b.topicname as child from topics as
a, topics as b where a.id = b.pid;
parent | child
--------+------------
Apache | PHP
News | Tech News
News | World News
(3 rows)
do you mean return also root nodes? You could just add
insert into topics (id, topicname) values (0, 'root');
and then you should be getting
test=# select a.topicname as parent, b.topicname as child from topics as
a, topics as b where a.id = b.pid;
parent | child
--------+------------
root | Apache
root | News
root | Foo
root | XHTML
Apache | PHP
News | Tech News
News | World News
or, if you dont want to add a ficticious root node you could do a left
join (if you can do left self joins, dont see a reason why not, but
never did it):
test=# select a.topicname as parent, b.topicname as child from topics as
a left join topics as b on a.id = b.pid;
parent | child
--------+------------
XHTML | null
Foo | null
Apache | PHP
News | Tech News
News | World News
but then, your self referencing querry starts going into the idea of
recursive select statements. I have little knowledge in this, MS-SQL
does not have true recursive selects (you can string up bunch of left
joins, but its a workaround hack). Oracle and I think db2 do support
recursive selects, but only to a certain level (Oracle recurses up to
32levels I think), I wouldnt mind hearing how recursive Select would
work in your case:
by recursive I mean I want to select all nodes who have a
specific node above the tree (be it parent/grand parent/
grand grand parent, etc).
/apz, If your aim in life is nothing, you can't miss.
