Question

I have a single product table with multiple fields which contain user evaluations of various attributes:

product  | attr_1_eval   | attr_2_eval   | attr_3_eval
ABC      | Correct       | Incorrect     | Null
DEF      | Incorrect     | Null          | Null
XYZ      | Undetermined  | Null          | Incorrect
123      | Null          | Undetermined  | Correct
456      | Incorrect     | Correct       | Correct

I need to write a query which totals up those attribute evaluations across all products (where not null):

evaluation  | correct   | incorrect   | undetermined
attr_1      | 1         | 2           | 1
attr_2      | 1         | 1           | 1
attr_3      | 2         | 1           | 0

This SQL gets me part way there:

SELECT 
SUM(CASE WHEN attr_1_eval = 'Correct' then 1 else 0 END) AS correct,
SUM(CASE WHEN attr_1_eval = 'Incorrect' then 1 else 0 END) AS incorrect,
SUM(CASE WHEN attr_1_eval = 'Undetermined' then 1 else 0 END) AS undetermined,
SUM(CASE WHEN attr_2_eval = 'Correct' then 1 else 0 END) AS correct,
...
FROM product

But it doesn’t group attr_1, attr_2.. by rows with error counts in the columns (as in the desired result set above). I’m using Postgres but help in any flavor of SQL would be most welcome.

Answer 1

Could you do 3 unions?

SELECT 
  'attr_1' AS evaluation,
  SUM(CASE WHEN attr_1_eval = 'Correct' then 1 else 0 END) AS correct,
  SUM(CASE WHEN attr_1_eval = 'Incorrect' then 1 else 0 END) AS incorrect,
  SUM(CASE WHEN attr_1_eval = 'Undetermined' then 1 else 0 END) AS undetermined
FROM product
UNION
SELECT 
  'attr_2' AS evaluation,
  SUM(CASE WHEN attr_2_eval = 'Correct' then 1 else 0 END) AS correct,
  SUM(CASE WHEN attr_2_eval = 'Incorrect' then 1 else 0 END) AS incorrect,
  SUM(CASE WHEN attr_2_eval = 'Undetermined' then 1 else 0 END) AS undetermined
FROM product
UNION
SELECT 
  'attr_3' AS evaluation,
  SUM(CASE WHEN attr_3_eval = 'Correct' then 1 else 0 END) AS correct,
  SUM(CASE WHEN attr_3_eval = 'Incorrect' then 1 else 0 END) AS incorrect,
  SUM(CASE WHEN attr_3_eval = 'Undetermined' then 1 else 0 END) AS undetermined
FROM product

It’s not the most elegant/efficient solution probably but it should get what you want