Re: Left Outer Join Question

On Wed, 27 Feb 2002, Edward Murray wrote:

> I seem to have reached the limits of my (postgres)SQL knowledge. If
> anybody could help me out of this predicament it would be greatly
> appreciated. I know the explanation of this problem may seem a bit complex
> but I am hoping the answer may be quite simple.
>
> I have items represented for sale on a web-site. I have a number of
> sections which each have sub-categories under them and each category may
> contain items.
>
> Each item is posted by a shop with a client number.
>
> I am trying to list the sub-categories of a section and the number of
> items posted in by a particular shop in each of those categories.
>
> Simplified versions of the tables are as such:
>
> sections (
>     recordnum int
>     name varchar
> )
>
> category (
>     recordnum int
>     name varchar
> )
>
>
>
> section_subcats (
>     sectionum int
>     catnum int
> )
>
> items (
>     recordnum int
>     catnum int
>     clientnum int
>     name varchar
> )
>
> The categories are assigned to the sections via the Section_subcats table
> using data in the tables as such:
>
>
> section
> 1     Fruit
>
> category
> 1    Apple
> 2    Pear
> 3    Orange
>
>
> section_subcats
> 1    1
> 1    2
> 1    3
>
> items
> 1    1    333    'Jonathan'
> 2    1    333    'Fuji'
> 3    1    444    'Granny Smith'
> I am trying to construct a query which will return something like the
> following for clientnum 333:
>
>
> Fruit
> -----
> Apple     2
> Pear     0
> Orange    0
>
> I have tried the following query but the results with no items are
> excluded:
>
> select category.name, count(items.recordnum) from category left outer join
> items on (category.recordnum = items.catnum) where category.recordnum =
> section_subcats.catnum and section_subcats.sectionnum = 1 and
> items.clientnum = 333 group by category.name;

Well, IIRC, by testing items.clientnum=333 you're getting rid of the
outerness of the join since those will be NULL in the rows so you're
just throwing them back out.

Maybe:
select category.name, count(foo.recordnum) from category inner join
section_subcats on (category.recordnum=section_subcats.catnum) left outer
join (select * from items where clientnum=333) foo on
(category.recordnum=foo.catnum) where section_subcats.sectionnum=1 group
by category.name;