SQL - Count occurrences across multiple columns

Question

I have a hypothetical database comprised of patients and their comorbidities (or diagnosis codes as they are often termed):

PATIENT_ID | HOSPITAL | DIAG_01 | DIAG_02 | DIAG_03 | DIAG_04 ...up to Diag_14
 Patient 1 |  Hosp1   |  C01    |  C02    |  NULL   |  NULL
 Patient 2 |  Hosp1   |  A01    |  A02    |  A03    |  P37
 Patient 3 |  Hosp2   |  A01    |  D10    |  X01    |  NULL

A patients comorbidities (there can be up to 14 of them recorded) define any illness/disease the patient may have. In particular C00 - C97 and D00 - D48 relate to Cancer ICD-10 codes.

I'd like to produce a summary for each Hospital which counts how many of their patients had a Cancer ICD-10 code in the diagnosis column.

Based on the example above, both Hosp1 and Hosp2 would have 1 patient each with Cancer. The C01 and C02 diagnosis codes for Patient 1 would only be counted as one patient with Cancer. However if Patient 1 was admitted again it would count as another case of a patient with Cancer.

I tried creating a table of all the Cancer comorbidities and using a JOIN query on all 14 of the diag columns but could not work out to do a count.

Any ideas or help would be massively, massively appreciated.

Answer 1

You have a bad data layout. You should have a junction table. But, you don't. You can do what you want with union all :

select hospital, count(distinct patient_id)
from ((select patient_id, hospital, diag_01 as diag
       from hypothetical
      ) union all
      (select patient_id, hospital, diag_02 as diag
       from hypothetical
      )
      union all
      . . .
      (select patient_id, hospital, diag_14 as diag
       from hypothetical
      )
     ) phd
where diag in ('C01', 'C02');

Answer 2

Use union all

SELECT * FROM /**customize your select here could be count() etc **/
 (SELECT PATIENT_ID,HOSPITAL,DIAGNOSIS
FROM (
    SELECT PATIENT_ID, HOSPITAL, LEFT(DIAG_01,1) AS DIAGNOSIS FROM table_name WHERE DIAG_01 IS NOT NULL UNION ALL   
    SELECT PATIENT_ID, HOSPITAL, LEFT(DIAG_02,1) AS DIAGNOSIS FROM table_name WHERE DIAG_02 IS NOT NULL UNION ALL
    SELECT PATIENT_ID, HOSPITAL, LEFT(DIAG_03,1) AS DIAGNOSIS FROM table_name WHERE DIAG_03 IS NOT NULL UNION ALL
    SELECT PATIENT_ID, HOSPITAL, LEFT(DIAG_04,1) AS DIAGNOSIS FROM table_name WHERE DIAG_04 IS NOT NULL
      ) AS T1
GROUP BY PATIENT_ID,HOSPITAL,DIAGNOSIS
) AS T2
WHERE DIAGNOSIS IN ('C','D') /**where **/