Re: How to join table to itself N times?

[AI Rumman <rummandba@xxxxxxxxx>]

On Wed, Mar 20, 2013 at 7:38 PM, W. Matthew Wilson <matt@xxxxxxxxxx> wrote:

I got this table right now:

select * from market_segment_dimension_values ;

+--------------------------+---------------+

| market_segment_dimension |     value     |

+--------------------------+---------------+

| geography                | north         |

| geography                | south         |

| industry type            | retail        |

| industry type            | manufacturing |

| industry type            | wholesale     |

+--------------------------+---------------+

(5 rows)

The PK is (market_segment_dimension, value).

The dimension column refers to another table called market_segment_dimensions.

So, "north" and "south" are to values for the "geography" dimension.

In that data above, there are two dimensions.  But sometimes there could be just one dimension, or maybe three, ... up to ten.

Now here's the part where I'm stumped.

I need to create a cartesian product of the dimensions.

I came up with this approach by hard-coding the different dimensions:

 with geog as (                                 

    select value

    from market_segment_dimension_values

    where market_segment_dimension = 'geography'),

    industry_type as (

    select value

    from market_segment_dimension_values

    where market_segment_dimension = 'industry type')

select geog.value as g,

industry_type.value as ind_type

from geog

cross join industry_type

;

+-------+---------------+

|   g   |   ind_type    |

+-------+---------------+

| north | retail        |

| north | manufacturing |

| north | wholesale     |

| south | retail        |

| south | manufacturing |

| south | wholesale     |

+-------+---------------+

(6 rows)

But that won't work if I add a new dimension (unless I update the query).  For example, maybe I need to add a new dimension called, say, customer size, which has values "big" and "small".  A

I've got some nasty plan B solutions, but I want to know if there's some solution.

There's a really elegant solution in python using itertools.product, like this:

>>> list(itertools.product(*[['north', 'south'], ['retail', 'manufacturing', 'wholesale']]))

[('north', 'retail'),

 ('north', 'manufacturing'),

 ('north', 'wholesale'),

 ('south', 'retail'),

 ('south', 'manufacturing'),

 ('south', 'wholesale')]

All advice is welcome.  Thanks in advance!

Matt

--
W. Matthew Wilson
matt@xxxxxxxxxx
http://tplus1.com

You may try:

Select a.value, b.value

from market_segment_dimension_values as a,

from market_segment_dimension_values as b

where a.market_segment_dimension <> b.market_segment_dimension

-- AI