Hi Bryan,
My current (maybe about to be deprecated now) category table is a recursive
table that I join on the line-item table. Your suggestion of using a table
for each level of category is probably a bit easier to manage in some
respects. However, as you pointed out, it becomes unwieldy when various
branches of the category tree have different lengths. One effectively has
to have the same number of levels in all branches. If only reality were so
refined! :-) That is why I chose the recursive table structure that I
gather is fairly common where the depth of a tree is not necessarily "even"
or known.
However, I like the view you created - I have not really used views very
much so far, so it is good to see this use case for a view. That said, now
that I have finally gotten the chance to try ltree, I think I like it a
lot. I wonder how portable it is, but it seems to do the trick extremely
well. Mind you, I am not necessarily looking to change DBMSes anytime
soon, but it is still good to know how to handle recursive/hierarchical
structures... ummm... the old-fashioned way(?). ;-)
I think using WITH RECURSIVE would probably do the trick for my recursive
category table, but - so far - the WITH construct makes my eyes glaze
over. I feel like I will have returned the *one ring* if I get that.
On Fri, Feb 22, 2013 at 11:54 PM, Bryan L Nuse <n...@uga.edu> wrote:
> <SNIP>
>
> Hello Don,
>
> Perhaps I am missing something about what your constraints are, or what
> you're trying to achieve, but is there any reason you could not use a
> series of joined tables indicating parent-child relationships? The
> following example follows that in your previous posts. Note that this
> approach (as given) will not work if branches stemming from the same node
> are different lengths. That is, if you have costs associated with
> "Transportation.Bicycle.Gear", you could not also have a category
> "Transportation.Bicycle.Gear.Chain_ring". (To add the latter category,
> you'd have to put costs from the former under something like
> "Transportation.Bicycle.Gear.General" -- or modify the approach.) However,
> lengthening the "Alcohol" branches, e.g., by tacking on a level5 table
> would be easy. Notice that level3 and level4 are not true look-up
> tables, since they may contain duplicate cat values.
>
> If I'm off base, by all means specify just how.
>
> Regards,
> Bryan
>
> --------------------------------------------------
>
> CREATE TABLE level1 (
> cat text PRIMARY KEY
> );
>
> CREATE TABLE level2 (
> cat text PRIMARY KEY,
> parent text REFERENCES level1(cat)
> );
>
> CREATE TABLE level3 (
> cat text,
> parent text REFERENCES level2(cat),
> cost numeric(6,2)
> );
>
> CREATE TABLE level4 (
> cat text,
> parent text,
> cost numeric(6,2)
> );
>
>
> INSERT INTO level1
> VALUES ('Transportation'),
> ('Groceries');
>
> INSERT INTO level2
> VALUES ('Auto', 'Transportation'),
> ('Bicycle', 'Transportation'),
> ('Fares', 'Transportation'),
> ('Food', 'Groceries'),
> ('Beverages', 'Groceries');
>
> INSERT INTO level3
> VALUES ('Fuel', 'Auto', 50.00),
> ('Maintenance', 'Auto', 30.00),
> ('Fuel', 'Auto', 25.00),
> ('Gear', 'Bicycle', 40.00),
> ('Gear', 'Bicycle', 20.00),
> ('Bus', 'Fares', 10.00),
> ('Train', 'Fares', 5.00),
> ('Beverages', 'Food', 30.00),
> ('Fruit_Veg', 'Food', 40.00),
> ('Meat_Fish', 'Food', 80.00),
> ('Grains_Cereals', 'Food', 30.00),
> ('Alcohol', 'Beverages', NULL),
> ('Juice', 'Beverages', 45.00),
> ('Other', 'Beverages', 15.00);
>
> INSERT INTO level4
> VALUES ('Beer', 'Alcohol', 25.00),
> ('Spirits', 'Alcohol', 10.00),
> ('Wine', 'Alcohol', 50.00);
>
>
> CREATE VIEW all_cats AS (
> SELECT a.cat AS level4,
> b.cat AS level3,
> c.cat AS level2,
> d.cat AS level1,
> CASE WHEN a.cost IS NULL THEN 0
> WHEN a.cost IS NOT NULL THEN a.cost
> END
> + CASE WHEN b.cost IS NULL THEN 0
> WHEN b.cost IS NOT NULL THEN b.cost
> END AS cost
> FROM level4 a
> FULL JOIN
> level3 b
> ON (a.parent = b.cat)
> FULL JOIN
> level2 c
> ON (b.parent = c.cat)
> FULL JOIN
> level1 d
> ON (c.parent = d.cat)
> ORDER BY level1, level2, level3, level4
> );
>
>
>
> SELECT * FROM all_cats;
>
> level4 | level3 | level2 | level1 | cost
> ---------+----------------+-----------+----------------+-------
> Beer | Alcohol | Beverages | Groceries | 25.00
> Spirits | Alcohol | Beverages | Groceries | 10.00
> Wine | Alcohol | Beverages | Groceries | 50.00
> | Juice | Beverages | Groceries | 45.00
> | Other | Beverages | Groceries | 15.00
> | Beverages | Food | Groceries | 30.00
> | Fruit_Veg | Food | Groceries | 40.00
> | Grains_Cereals | Food | Groceries | 30.00
> | Meat_Fish | Food | Groceries | 80.00
> | Fuel | Auto | Transportation | 50.00
> | Fuel | Auto | Transportation | 25.00
> | Maintenance | Auto | Transportation | 30.00
> | Gear | Bicycle | Transportation | 20.00
> | Gear | Bicycle | Transportation | 40.00
> | Bus | Fares | Transportation | 10.00
> | Train | Fares | Transportation | 5.00
> (16 rows)
>
>
>
>
> SELECT level1,
> count(cost) AS num_branches,
> sum(cost) AS total_cost
> FROM all_cats
> GROUP BY level1
> ORDER BY level1;
>
> level1 | num_branches | total_cost
> ----------------+--------------+------------
> Groceries | 9 | 325.00
> Transportation | 7 | 180.00
> (2 rows)
>
>
--
D.C. Parris, FMP, Linux+, ESL Certificate
Minister, Security/FM Coordinator, Free Software Advocate
http://dcparris.net/
<https://www.xing.com/profile/Don_Parris><http://www.linkedin.com/in/dcparris>
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