Question

I have a table with data that looks like this:

create table demo_patient_info (
  attend timestamp,  
  patient_id int, 
  blood_pressure double
);

I would like to write (preferably ANSI) SQL queries that allow me to do the following:

query1:

return the difference between bp of all patients with each other (using a WHERE clause to restrict the number of rows returned in the cartesian product)

query2:

return the difference between bp of each patient and one specific (i.e. specified) patient in the table

Answer 1

1)

SELECT 
    t1.patient_id
   ,t2.patient_id
   ,t1.blood_pressure - t2.blood_pressure as bp_diff
FROM
   demo_patient_info t1
CROSS JOIN
   demo_patient_info t2
WHERE
   t1.patient_id < t2.patient_id

2)

SELECT 
    t1.patient_id
   ,t2.patient_id
   ,t1.blood_pressure - t2.blood_pressure as bp_diff
FROM
   demo_patient_info t1
CROSS JOIN
   demo_patient_info t2
WHERE 
   t2.patient_id = 1

This will likely give two rows for each match though, which may or may not be OK

edit: WHERE t1.patient_id < t2.patient_id in query 1 is there to prevent the following, duplication and self matching

ID1 ID2  diff 
1    2    1.4 
2    1    1.4 
1    1     0 

thanks to Jonathan Leffler for the hint 🙂