Quoth "David Johnston" <pol...@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
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