The categories table:
=# \d
List of relations
Schema | Name | Type | Owner
--------+-------------+-------+-------
public | categories | table | pgsql
public | products | table | pgsql
public | ticketlines | table | pgsql
(3 rows)
Contents of categories:
=# select * from categories;
id | name | parentid
----+--------+----------
1 | Rack |
2 | Women | 1
3 | Shorts | 2
4 | Wares |
5 | Toys | 4
6 | Trucks | 5
(6 rows)
Running the following query:
WITH RECURSIVE nodes_cte(name, id, parentid, depth, path) AS (
-- Base case?
SELECT c.name,
c.id,
c.parentid,
1::INT AS depth,
c.id::TEXT AS path
FROM categories c
WHERE c.parentid = ''
UNION ALL
-- nth case
SELECT c.name,
c.id,
c.parentid,
n.depth + 1 AS depth,
(n.path || '->' || c.id::TEXT)
FROM nodes_cte n
JOIN categories c on n.id = c.parentid
)
SELECT * FROM nodes_cte AS n GROUP BY n.name, n.id, n.parentid, n.depth, n.path ORDER BY n.id ASC
;
yields these results:
name | id | parentid | depth | path
--------+----+----------+-------+---------
Rack | 1 | | 1 | 1
Women | 2 | 1 | 2 | 1->2
Shorts | 3 | 2 | 3 | 1->2->3
Wares | 4 | | 1 | 4
Toys | 5 | 4 | 2 | 4->5
Trucks | 6 | 5 | 3 | 4->5->6
(6 rows)
Great!
But given a similar table (categories):
=# \d categories
Table "public.categories"
Column | Type | Modifiers
----------+-------------------+-----------
id | character varying | not null
name | character varying | not null
parentid | character varying |
image | bytea |
Indexes:
"categories_pkey" PRIMARY KEY, btree (id)
"categories_name_inx" UNIQUE, btree (name)
Referenced by:
TABLE "products" CONSTRAINT "products_fk_1" FOREIGN KEY (category) REFERENCES categories(id)
=# select * from categories;
id | name | parentid | image
--------------------------------------+-------+--------------------------------------+-------
611572c9-326d-4cf9-ae4a-af5269fc788e | Rack | |
22d15300-40b5-4f43-a8d1-902b8d4c5409 | Women | 611572c9-326d-4cf9-ae4a-af5269fc788e |
6b061073-96f4-49a1-9205-bab7c878f0cf | Wares | |
3f018dfb-e6ee-40d1-9dbc-31e6201e7625 | Toys | 6b061073-96f4-49a1-9205-bab7c878f0cf |
(4 rows)
the same query produces zero rows.
Why?
Is it something to do with primary / foreign keys?
WHERE COALESCE(parent_id, '') = ''
Worked. Thank you.
Related
Consider the following sample from the table category:
category_id | category_name | super_category
-------------+----------------------------------------------------------------+----------------
1 | Features +|
| |
2 | Alle SACDs +| 1
| |
3 | Formate +|
| |
4 | Box-Sets +| 3
| |
5 | Action, Thriller & Horror +| 4
| |
6 | Alternative +| 4
| |
7 | Blues +| 4
| |
8 | Country +| 4
| |
9 | Alternative Country & Americana& Country | 8
10 | Bestseller& Country | 8
11 | Bluegrass& Country | 8
...
The column super_category lists the immediate parent category for any given category. When super_category is NULL, that category is a main category.
Table with all of the main categories:
SELECT * FROM category WHERE super_category IS NULL;
category_id | category_name | super_category
-------------+----------------+----------------
1 | Features +|
| |
3 | Formate +|
| |
497 | Formats +|
| |
544 | Genres +|
| |
923 | Interpreten +|
| |
941 | Kategorien +|
| |
19208 | Schauspieler +|
| |
19211 | Shop-überblick+|
| |
19502 | Shops +|
| |
21350 | Subjects +|
| |
21513 | Unter 10 EUR +|
| |
21520 | Unter 15 EUR +|
I need to write a recursive query that outputs a table that lists all the categories and which main categories the belong to.
So far I only have the following:
WITH RECURSIVE query AS (
SELECT cat.category_id, cat.super_category
FROM category cat
WHERE cat.super_category IS NULL
UNION ALL
SELECT query.category_id, cat.super_category
FROM query JOIN category cat on cat.super_category = query.category_id
)
SELECT * FROM query;
The logic is as follows:
We define the base case where the category is a main category (super_category IS NULL)
Then we define the recursive case, but I am not sure how I should define it.
Any suggestions?
You are in the correct path with the use of a recursive CTE. You can do:
with recursive
n as (
select category_id, category_name, super_category, 0 as level
from category
union all
select n.category_id, n.category_name, c.super_category, n.level + 1
from n
join category c on c.category_id = n.super_category
)
select category_id, category_name, super_category
from n
where (category_id, level) in (
select distinct on (category_id) category_id, level
from n
order by category_id, level desc
)
How can I use string_to_array or split_part on another column value.
I want do something like select * from tenants where id IN (select string_to_array(select ancestry from tenants where id = 39,'/'));
-[ RECORD 1 ]-------------+----------------------
id | 1
domain |
subdomain |
name | My Company
login_text |
logo_file_name |
logo_content_type |
logo_file_size |
logo_updated_at |
login_logo_file_name |
login_logo_content_type |
login_logo_file_size |
login_logo_updated_at |
ancestry |
divisible | t
description | Tenant for My Company
use_config_for_attributes | t
default_miq_group_id | 1
source_type |
source_id |
-[ RECORD 3 ]-------------+----------------------
id | 35
domain |
subdomain |
name | Tenant_2
login_text |
logo_file_name |
logo_content_type |
logo_file_size |
logo_updated_at |
login_logo_file_name |
login_logo_content_type |
login_logo_file_size |
login_logo_updated_at |
ancestry | 1
divisible | t
description | Tenant_2
use_config_for_attributes | f
default_miq_group_id | 36
source_type |
source_id |
-[ RECORD 7 ]-------------+----------------------
id | 39
domain |
subdomain |
name | Child_Teanant_202
login_text |
logo_file_name |
logo_content_type |
logo_file_size |
logo_updated_at |
login_logo_file_name |
login_logo_content_type |
login_logo_file_size |
login_logo_updated_at |
ancestry | 1/35
divisible | t
description | Child_Teanant_202
use_config_for_attributes | f
default_miq_group_id | 52
source_type |
source_id |
Use regex to enforce word boundaries:
select *
from tenants
where (select ancestry from tenants where id = 39)
~ ('\y' || id || '\y')
See live demo.
Without the word boundaries an id of 1 would match an ancestry of 123.
Note Postgres's unusual regex for word boundary \y, which elsewhere is \b.
There are two ways to solve this.
One is to simply unnest the elements of ancestry
select *
from tenants
where id in (select a.id::int
from tenants t2
cross join unnest(string_to_array(t2.ancestry, '/')) as a(id)
where t2.id = 39);
Converting the string to an array in order to be able to use the = ANY() operator is a bit tricky, because you need two levels of parentheses plus a type cast to an integer array to make that work:
select *
from tenants
where id = any ((select string_to_array(t2.ancestry, '/')
from tenants t2
where t2.id = 39)::int[]);
Online example
I have three main tables meetings, persons, hobbies with two relational tables.
Table meetings
+---------------+
| id | subject |
+----+----------+
| 1 | Kickoff |
| 2 | Relaunch |
| 3 | Party |
+----+----------+
Table persons
+------------+
| id | name |
+----+-------+
| 1 | John |
| 2 | Anna |
| 3 | Linda |
+----+-------+
Table hobbies
+---------------+
| id | name |
+----+----------+
| 1 | Soccer |
| 2 | Tennis |
| 3 | Swimming |
+----+----------+
Relation Table meeting_person
+-----------------+-----------+
| id | meeting_id | person_id |
+----+------------+-----------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 3 | 1 |
+----+------------+-----------+
Relation Table person_hobby
+----------------+----------+
| id | person_id | hobby_id |
+----+-----------+----------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 3 | 1 |
+----+-----------+----------+
Now I want to to find the common hobbies of all person attending each meeting.
So the desired result would be:
+------------+-----------------+------------------------+
| meeting_id | persons | common_hobbies |
| | (Aggregated) | (Aggregated) |
+------------+-----------------+------------------------+
| 1 | John,Anna,Linda | Soccer |
| 2 | John,Anna | Soccer,Tennis |
| 3 | John | Soccer,Tennis,Swimming |
+------------+-----------------+------------------------+
My current work in progress is:
select
m.id as "meeting_id",
(
select string_agg(distinct p.name, ',')
from meeting_person mp
inner join persons p on mp.person_id = p.id
where m.id = mp.meeting_id
) as "persons",
string_agg(distinct h2.name , ',') as "common_hobbies"
from meetings m
inner join meeting_person mp2 on m.id = mp2.meeting_id
inner join persons p2 on mp2.person_id = p2.id
inner join person_hobby ph2 on p2.id = ph2.person_id
inner join hobbies h2 on ph2.hobby_id = h2.id
group by m.id
But this query lists not the common_hobbies but all hobbies which are at least once mentioned.
+------------+-----------------+------------------------+
| meeting_id | persons | common_hobbies |
+------------+-----------------+------------------------+
| 1 | John,Anna,Linda | Soccer,Tennis,Swimming |
| 2 | John,Anna | Soccer,Tennis,Swimming |
| 3 | John | Soccer,Tennis,Swimming |
+------------+-----------------+------------------------+
Does anyone have any hints for me, on how I could solve this problem?
Cheers
This problem can be solved by implement custom aggregation function (found it here):
create or replace function array_intersect(anyarray, anyarray)
returns anyarray language sql
as $$
select
case
when $1 is null then $2
when $2 is null then $1
else
array(
select unnest($1)
intersect
select unnest($2))
end;
$$;
create aggregate array_intersect_agg (anyarray)
(
sfunc = array_intersect,
stype = anyarray
);
So, the solution can be next:
select
meeting_id,
array_agg(ph.name) persons,
array_intersect_agg(hobby) common_hobbies
from meeting_person mp
join (
select p.id, p.name, array_agg(h.name) hobby
from person_hobby ph
join persons p on ph.person_id = p.id
join hobbies h on h.id = ph.hobby_id
group by p.id, p.name
) ph on ph.id = mp.person_id
group by meeting_id;
Look the example fiddle
Result:
meeting_id | persons | common_hobbies
-----------+-----------------------+--------------------------
1 | {John,Anna,Linda} | {Soccer}
3 | {John} | {Soccer,Tennis,Swimming}
2 | {John,Anna} | {Soccer,Tennis}
I have a table as such (tbl):
+----+------+-----+
| pk | attr | val |
+----+------+-----+
| 0 | ohif | 4 |
| 1 | foha | 56 |
| 2 | slns | 2 |
| 3 | faso | 11 |
+----+------+-----+
And another table in n-to-1 relationship with tbl (tbl2):
+----+-----+
| pk | rel |
+----+-----+
| 0 | 0 |
| 1 | 1 |
| 2 | 0 |
| 3 | 2 |
| 4 | 2 |
| 5 | 3 |
| 6 | 1 |
| 7 | 2 |
+----+-----+
(tbl2.rel -> tbl.pk.)
I would like to select only the rows from tbl which are in relationship with at least n rows from tbl2.
I.e., for n = 2, I want this table:
+----+------+-----+
| pk | attr | val |
+----+------+-----+
| 0 | ohif | 4 |
| 1 | foha | 56 |
| 2 | slns | 2 |
+----+------+-----+
This is the solution I came up with:
SELECT DISTINCT ON (tbl.pk) tbl.*
FROM (
SELECT tbl.pk
FROM tbl
RIGHT OUTER JOIN tbl2 ON tbl2.rel = tbl.pk
GROUP BY tbl.pk
HAVING COUNT(tbl2.*) >= 2 -- n
) AS tbl_candidates
LEFT OUTER JOIN tbl ON tbl_candidates.pk = tbl.pk
Can it be done without selecting the candidates with a subquery and re-joining the table with itself?
I'm on Postgres 10. A standard SQL solution would be better, but a Postgres solution is acceptable.
OK, just join once, as below:
select
t1.pk,
t1.attr,
t1.val
from
tbl t1
join
tbl2 t2 on t1.pk = t2.rel
group by
t1.pk,
t1.attr,
t1.val
having(count(1)>=2) order by t1.pk;
pk | attr | val
----+------+-----
0 | ohif | 4
1 | foha | 56
2 | slns | 2
(3 rows)
Or just join once and use CTE(with clause), as below:
with tmp as (
select rel from tbl2 group by rel having(count(1)>=2)
)
select b.* from tmp t join tbl b on t.rel = b.pk order by b.pk;
pk | attr | val
----+------+-----
0 | ohif | 4
1 | foha | 56
2 | slns | 2
(3 rows)
Is the SQL clearer?
I am working with PostgreSQL 9.3, and I have this:
PARENT_TABLE
ID | NAME
1 | N_A
2 | N_B
3 | N_C
CHILD_TABLE
ID | PARENT_TABLE_ID | KEY | VALUE
1 | 1 | K_A | V_A
2 | 1 | K_B | V_B
3 | 1 | K_C | V_C
5 | 2 | K_A | V_D
6 | 2 | K_C | V_E
7 | 3 | K_A | V_F
8 | 3 | K_B | V_G
9 | 3 | K_C | V_H
Note that I might add K_D in KEY's, it's completely dynamic.
What I want is a query that returns me the following:
QUERY_TABLE
ID | NAME | K_A | K_B | K_C | others K_...
1 | N_A | V_A | V_B | V_C | ...
2 | N_B | V_D | | V_E | ...
3 | N_C | V_F | V_G | V_H | ...
Is this possible to do ? If so, how ?
Since there can be values missing, you need the "safe" form of crosstab() with the column names as second parameter:
SELECT * FROM crosstab(
'SELECT p.id, p.name, c.key, c."value"
FROM parent_table p
LEFT JOIN child_table c ON c.parent_table_id = p.id
ORDER BY 1'
,$$VALUES ('K_A'::text), ('K_B'), ('K_C')$$)
AS t (id int, name text, k_a text, k_b text, k_c text; -- use actual data types
Details in this related answer:
PostgreSQL Crosstab Query
About adding "extra" columns:
Pivot on Multiple Columns using Tablefunc