Re: Crosstab-style query in pure SQL

Richard,

> I have a table containing milestones achieved for projects, marked as
> being in particular quarters.
>
> CREATE TABLE milestones (
> proj_id int4,
> sortorder SERIAL,
> qtr int4,
> description varchar(200)
> );
>
> Now I need the milestone descriptions output across the page like:
> proj_id | q1 | q2 | q3 | q4
> and sorted according to "sortorder".

Ah! A classic SQL problem.

Take a look at Joe Celko's "SQL for Smarties": he does a good job of
defining and discussing the three different solutions to the "Crosstab
Query" problem.

A second method you can use is the subselect method:

SELECT proj_id, sortorder, qart1.q1, quart2.q2 ....
FROM milestones
LEFT OUTER JOIN ( SELECT proj_id, description as q1
FROM milestones WHERE qtr = 1) quart1
ON quart1.proj_id = milestones.proj_id
LEFT OUTER JOIN ( SELECT proj_id, description q2 ...

However, the above is generally chosen over the CASE statement method
when the crosstab involves multiple tables; in your case, it is not a
performance or similicity gain.

There is a third method which involves building a grid of values:

ref qtr1 qtr2 qtr3 qtr4
qtr1 1 0 0 0
qtr2 0 1 0 0
qtr3 0 0 1 0
qtr4 0 0 0 1

This method is the only one of the three that prevents you from having
to build your query using procedural logic for a dynamic set of
parameters. However, the above is set up for numeric values (i.e. one
multiplies the grid number by the value to add into calculations) and
will not work for your varchar-returning query.

You could, however, constuct the grid with NULLs and '' strings and use
that to modify the value; I leave that to your ingenuity.

-Josh Berkus