Count descendants in hierarchical query - php

Question

I was told that PostgreSQL is a better choice than MySQL for displaying hierarchical data, so I installed PostgreSQL and I'm ready to go.
This is the schema from my title (copied from pgAdmin):
CREATE TABLE public.gz_life_mammals (
id integer NOT NULL,
taxon text NOT NULL,
parent text NOT NULL,
parent_id smallint NOT NULL,
slug text,
name_common text,
plural text,
extinct smallint NOT NULL,
rank smallint NOT NULL,
key smallint NOT NULL,
CONSTRAINT "Primary Key" PRIMARY KEY (id)
);
This is my database connection and first query:
$dbh = pg_connect("host=localhost dbname=geozoo user=postgres");
if (!$dbh) {
die("Error in connection: " . pg_last_error());
}
$sql = "SELECT * FROM gz_life_mammals";
$result = pg_query($dbh, $sql);
while ($row = pg_fetch_array($result)) {
echo "ID: " . $row[0] . " | ";
echo "Taxon: " . $row[1] . " | ";
echo "ParentID: " . $row[3] . "<br>";
}
// free memory
pg_free_result($result);
// close connection
pg_close($dbh);
The most important table fields for this exercise are the first four (id, taxon, parent and parent_id. The data looks like this:
ID | TAXON | PARENT | PARENT_ID
1 | Mammalia | Chordata | 1
2 | Carnivora | Mammalia | 2
3 | Canidae | Carnivora | 3
4 | Canis | Canidae | 4
5 | Canis-lupus | Canis | 5
6 | Canis-latrans | Canis | 5
Where the last two rows represent the wolf (Canis lupus) and coyote (Canis latrans). Eventually, I'd like to be able to display the names of children, grandchildren, parents, great grandparents, etc. But right now I'm just trying to display the number of descendants. For example, if I navigated to MySite/life/mammalia, I might see the following display:
Orders: 19
Families: 58
Genera: 688
Species: 8,034
If I navigated to MySite/life/canidae, it might display something like this:
Genera: 6
Species: 37
Can anyone show me the best way to write that kind of query and display the results (with PHP)?

Answer 1

Given the table:
select * from gz_life_mammals;
id | taxon | parent | parent_id
----+---------------+-----------+-----------
1 | Mammalia | Chordata | 1
2 | Carnivora | Mammalia | 2
3 | Canidae | Carnivora | 3
4 | Canis | Canidae | 4
5 | Canis-lupus | Canis | 5
6 | Canis-latrans | Canis | 5
(6 rows)
and the function to translate parent_id into taxonomic rank name:
create function tax_rank(id integer) returns text as $$
select case id
when 1 then 'Classes'
when 2 then 'Orders'
when 3 then 'Families'
when 4 then 'Genera'
when 5 then 'Species'
end;
$$ language sql;
you can query number of descendants with the following recursive query:
with recursive hier(taxon,parent_id) as (
select m.taxon, null::integer
from gz_life_mammals m
where taxon='Mammalia' --<< substitute me
union all
select m.taxon, m.parent_id
from hier, gz_life_mammals m
where m.parent=hier.taxon
)
select tax_rank(parent_id),
count(*) num_of_desc
from hier
where parent_id is not null
group by parent_id
order by parent_id;
tax_rank | num_of_desc
----------+-------------
Orders | 1
Families | 1
Genera | 1
Species | 2
(4 rows)
The interesting part is inside with recursive. The first part of the query selects the root row(s) of hierarchy. The second part (after union all) is called recursively and each time adds direct descendants to the previous result set. Read this to understand how it works in details.
After hierarchy is constructed, it can be represented as you like. In the above example only number of descendants are shown. You can get names as well:
with recursive hier(taxon,parent_id) as (
...
)
select tax_rank(parent_id),
taxon as name
from hier
where parent_id is not null
order by parent_id;
tax_rank | name
----------+---------------
Orders | Carnivora
Families | Canidae
Genera | Canis
Species | Canis-lupus
Species | Canis-latrans
(5 rows)
The same on one line:
with recursive hier(taxon,parent_id) as (
...
)
select tax_rank(parent_id),
string_agg(taxon,', ') as names
from hier
where parent_id is not null
group by parent_id
order by parent_id;
tax_rank | names
----------+----------------------------
Orders | Carnivora
Families | Canidae
Genera | Canis
Species | Canis-lupus, Canis-latrans
(4 rows)
And so on...