Обсуждение: Help needed creating a view

Hi list,

Given an 'applications' table for a static set of courses::

     user_id (integer)
 course_name (text)
   completed (boolean)

how best should I go about creating an 'alumni' view with columns:

 user_id (integer)
   maths (boolean)
 english (boolean)
       . .
       . .
       . .

where each of the columns (apart from user_id) is a boolean value representing
whether or not user_id completed each course?

Sebastian
--
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Lightweight, full-featured and mindful of your idyllic happiness.
http://home.gna.org/eap

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Sebastian Tennant
Sent: Thursday, January 26, 2012 6:55 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Help needed creating a view

Hi list,

Given an 'applications' table for a static set of courses::

     user_id (integer)
 course_name (text)
   completed (boolean)

how best should I go about creating an 'alumni' view with columns:

 user_id (integer)
   maths (boolean)
 english (boolean)
       . .
       . .
       . .

where each of the columns (apart from user_id) is a boolean value
representing whether or not user_id completed each course?

Sebastian
-

----------------------------------------------------------------------------
------------

A) SELECT user_id, CASE WHEN course_name = 'Maths' THEN completed ELSE false
END math_cmp, CASE WHEN course_name = 'English' THEN completed ELSE false
END AS english_cmp .... FROM applications
a) Expand to multiple columns and store either the default "false" or the
value of "completed" into the value for the corresponding column

B) SELECT user_id, CASE WHEN bool_or(math_cmp) THEN true ELSE false END AS
did_math, CASE WHEN bool_or(english_cmp) THEN true ELSE false END AS
did_english FROM  "A" GROUP BY user_id
b) Then determine whether the user_id has at least one "true" in the given
column by using the "bool_or" function

Dynamic columns are difficult to code in SQL.  You should probably also
include some kind of "OTHER COMPLETED DISCIPLINES" column to catch when you
add an previously unidentified course - "course_name NOT IN
('Maths','English','...')"

Also concerned with the fact that, as coded, a single complete course
triggers the given flag.  What happens when you want to specify that they
have only completed 3 of 4 courses?  Also, instead of hard-coding the
"course_name" targets you may want to do something like "CASE WHEN
course_name IN (SELECT course_name FROM courses WHERE course_type =
'Maths')".

David J.

Quoth "David Johnston" <polobo@yahoo.com>:
> A) SELECT user_id, CASE WHEN course_name = 'Maths' THEN completed ELSE false
> END math_cmp, CASE WHEN course_name = 'English' THEN completed ELSE false
> END AS english_cmp .... FROM applications
> a) Expand to multiple columns and store either the default "false" or the
> value of "completed" into the value for the corresponding column
>
> B) SELECT user_id, CASE WHEN bool_or(math_cmp) THEN true ELSE false END AS
> did_math, CASE WHEN bool_or(english_cmp) THEN true ELSE false END AS
> did_english FROM  "A" GROUP BY user_id
> b) Then determine whether the user_id has at least one "true" in the given
> column by using the "bool_or" function
>
> Dynamic columns are difficult to code in SQL.  You should probably also
> include some kind of "OTHER COMPLETED DISCIPLINES" column to catch when you
> add an previously unidentified course - "course_name NOT IN
> ('Maths','English','...')"
>
> Also concerned with the fact that, as coded, a single complete course
> triggers the given flag.  What happens when you want to specify that they
> have only completed 3 of 4 courses?  Also, instead of hard-coding the
> "course_name" targets you may want to do something like "CASE WHEN
> course_name IN (SELECT course_name FROM courses WHERE course_type =
> 'Maths')".

Many thanks David for a clear and comprehensive reply, although I haven't
completely grokked your use of bool_or.

No matter though, because 'CASE WHEN ... THEN <column_name> END' is precisely
the idiom I was looking for.

My view definition now looks something like this:

 CREATE VIEW alumni AS
   SELECT * FROM (
       -- query includes every user_id in applications
       SELECT user_id,
              CASE WHEN course_name='Maths'   THEN completed END AS maths_alumni,
              CASE WHEN course_name='English' THEN completed END AS english_alumni,
              ...
              ...
         FROM applications ) AS foo
    -- so we need to exclude user_ids who did not complete *any* courses
    WHERE  maths_alumni   IS TRUE
       OR  english_alumni IS TRUE
       ...
       ...;

Thanks again.

Sebastian
--
Emacs' AlsaPlayer - Music Without Jolts
Lightweight, full-featured and mindful of your idyllic happiness.
http://home.gna.org/eap