Re: [GENERAL] [BUGS] Postgresql query HAVING do not work

On 1/4/17, Gwork <nnj@riseup.net> wrote:
> On 1/5/17 2:22 AM, Vitaly Burovoy wrote:
>> On 1/4/17, Vitaly Burovoy <vitaly.burovoy@gmail.com> wrote:
>>> On 1/4/17, Gwork <nnj@riseup.net> wrote:
>>>> Version: Postgresql 9.5
>>>> OS: Debian 8 jessie run on docker
>>>>
>>>> Following this tutorial The Nested Set Model on
>>>> http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/
>>>>
>>>>
>>>> Section: Depth of a Sub-Tree.
>>>> SELECT node.name, (COUNT(parent.name) - (sub_tree.depth + 1)) AS depth
>>>> FROM nested_category AS node,
>>>>         nested_category AS parent,
>>>>         nested_category AS sub_parent,
>>>>         (
>>>>                 SELECT node.name, (COUNT(parent.name) - 1) AS depth
>>>>                 FROM nested_category AS node,
>>>>                 nested_category AS parent
>>>>                 WHERE node.lft BETWEEN parent.lft AND parent.rgt
>>>>                 AND node.name = 'PORTABLE ELECTRONICS'
>>>>                 GROUP BY node.name, node.lft
>>>>                 ORDER BY node.lft
>>>>         )AS sub_tree
>>>> WHERE node.lft BETWEEN parent.lft AND parent.rgt
>>>>         AND node.lft BETWEEN sub_parent.lft AND sub_parent.rgt
>>>>         AND sub_parent.name = sub_tree.name
>>>> GROUP BY node.name, node.lft, sub_tree.depth
>>>> ORDER BY node.lft;
>>>> +----------------------+---------+
>>>> | name                 |   depth |
>>>> |----------------------+---------|
>>>> | PORTABLE ELECTRONICS |       0 |
>>>> | MP3 PLAYERS          |       1 |
>>>> | FLASH                |       2 |
>>>> | CD PLAYERS           |       1 |
>>>> | 2 WAY RADIOS         |       1 |
>>>> +----------------------+---------+
>>>>
>>>>
>>>> Section: Find the Immediate Subordinates of a Node.
>>>> SELECT node.name, (COUNT(parent.name) - (sub_tree.depth + 1)) AS depth
>>>> FROM nested_category AS node,
>>>>         nested_category AS parent,
>>>>         nested_category AS sub_parent,
>>>>         (
>>>>                 SELECT node.name, (COUNT(parent.name) - 1) AS depth
>>>>                 FROM nested_category AS node,
>>>>                 nested_category AS parent
>>>>                 WHERE node.lft BETWEEN parent.lft AND parent.rgt
>>>>                 AND node.name = 'PORTABLE ELECTRONICS'
>>>>                 GROUP BY node.name, node.lft
>>>>                 ORDER BY node.lft
>>>>         )AS sub_tree
>>>> WHERE node.lft BETWEEN parent.lft AND parent.rgt
>>>>         AND node.lft BETWEEN sub_parent.lft AND sub_parent.rgt
>>>>         AND sub_parent.name = sub_tree.name
>>>> GROUP BY node.name, node.lft, sub_tree.depth
>>>> HAVING depth <= 1
>>>> ORDER BY node.lft;
>>>> Adding 'HAVING depth <= 1' to the query still return the same results
>>>> as
>>>> above instead of this:
>>>> +----------------------+---------+
>>>> | name                 |   depth |
>>>> |----------------------+---------|
>>>> | PORTABLE ELECTRONICS |       0 |
>>>> | MP3 PLAYERS          |       1 |
>>>> | FLASH                |       1 |
>>>> | CD PLAYERS           |       1 |
>>>> | 2 WAY RADIOS         |       1 |
>>>> +----------------------+---------+
>>>>
>>>> I don't know if I'm doing anything wrong?
>>>>
>>>> Note: Edit the post query by adding node.lft, sub_tree.depth to the
>>>> GROUP BY.
>>> Hello, Gwork,
>>>
>>> HAVING works fine, it is just confusing because of naming. HAVING
>>> works with column names from sources (which is "sub_tree.depth" in
>>> your example), not with names of final columns (because they get
>>> aliases later).
>>>
>>> You can check it adding depth to your SELECT part:
>>> SELECT node.name, (COUNT(parent.name) - (sub_tree.depth + 1)) AS depth
>>>     ,array_agg(depth)
>>> FROM nested_category AS node,
>>> ...
>>>
>>> and you can see that values there are not bigger than 1.
>>>
>>> You must use the same expression in HAVING clause as in SELECT one to
>>> get what you want:
>>> HAVING (COUNT(parent.name) - (sub_tree.depth + 1)) <= 1
>>>
>>> but the result will not have "FLASH" because it has "2" even in your
>>> example.
>>> +----------------------+-------+
>>> |         name         | depth |
>>> +----------------------+-------+
>>> | PORTABLE ELECTRONICS |     0 |
>>> | MP3 PLAYERS          |     1 |
>>> | CD PLAYERS           |     1 |
>>> | 2 WAY RADIOS         |     1 |
>>> +----------------------+-------+
>>> (4 rows)
>> I'm sorry, forgot to mention: If you want to deal with hierarchical
>> data, Postgres has better solution - recursive query[1]. When you
>> understand principles, it will be much easier for you to write queries
>> instead of mentioned in the article.
>>
>> For example, "Retrieving a Single Path" from "Adjacency model" can be
>> written as:
>> WITH RECURSIVE
>> sel(name, parent, depth) AS (
>>     SELECT name, parent, 0 FROM category WHERE name='FLASH'
>>     UNION ALL
>>     SELECT c.name, c.parent, depth + 1 FROM category c, sel WHERE
>> c.category_id=sel.parent
>> )
>> SELECT name FROM sel
>> ORDER BY depth DESC;
>>
>> which gives the same result and not depends on "parent.lft" which
>> don't have to increase.
>>
>> Moreover, you don't need to lock a table when you change data and you
>> can even add a constraint to keep consistency:
>> ALTER TABLE category ADD FOREIGN KEY (parent) REFERENCES
>> category(category_id) ON UPDATE CASCADE ON DELETE RESTRICT;
>>
>> [1]https://www.postgresql.org/docs/current/static/queries-with.html
>>
>
> Hi Vitaly,
>
> Your first solution worked great!
>
> I'll like try your second suggestion, I feel is gonna be a better solution
> very important to eliminate lock while updating table.
>
> I'll keep you posted if I have any further issue relating to the query.
>
> Thank you for helping out.

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P.S. Moved from -bugs[2] to -general.

[2]https://www.postgresql.org/message-id/flat/7582ea1e-6146-fd8d-b564-c2fe251210b2%40riseup.net
--
Best regards,
Vitaly Burovoy