Question

I’ve read a lot of the examples on self join, but they don’t seem to cover the case where some fields are not in some rows.

For eg, I have a database with:

testId, testItem, testResult

And the rows:

 1,test1,1
 1,test2,0
 1,test3,1
 2,test1,0
 2,test4,1
 2,test5,1

I would like the output:

testItem,a.testId,b.testId,a.testResult,b.testResult
test1,1,2,1,0
test2,1,NULL,0,NULL
test3,1,NULL,1,NULL
test4,NULL,2,NULL,1
test5,NULL,2,NULL,1

Essentially, I want to compare each testItem (test1->test5) from two different testIds (1 and 2) and compare their testResult values, factoring in testIds that may not have the same test Items.

Answer 1

Given your exact requirement, you can try this:

select testItem
     , max(case when testID = 1 then testID else null end) as testID1
     , max(case when testID = 2 then testID else null end) as testID2
     , max(case when testID = 1 then testResult else null end) as testResult1
     , max(case when testID = 2 then testResult else null end) as testResult2
from mytable
where testID in (1,2)
group by testItem

This makes a lot of assumptions about your data, so take it with a grain of salt.