Member Avatar for bajanstar

Hi Guys,

This is my problem:

In the below table:


donor receptor
1 -> 4
2 -> 5
4 -> 6

The above reads as follows...

Student 1 donates all of his/her money to Student 4
Student 2 donates all of his/her money to Student 5
Student 4 donates all of his/her money to Student 6

I would like to pull all of the ultimate donors from the above table. An ultimate donor is someone who is at all times a receptor and never a donor.

So in the above scenario Students 5 and 6 are ultimate donors. Does anyone have any ideas on how this can be accomplished?

I will be very appreciative of any inputs.

Thanks

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  • Latest Post Latest Post by tesuji

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Member Avatar for tesuji

Hello bajanstar

you may try this statement:

select receptor as 
  'ultimate donors, really? Shouldn''t it be ultimate receptors?' 
      from inthebelowtable where receptor NOT IN 
          (select donor from inthebelowtable)

krs,
tesu

Member Avatar for bajanstar

hi tesu,
You are correct. It is suppose to be ultimate receptor. Thanks for you input.

so what do you think about this solution

select distinct receptor
from yourtable a
left join (select donor from yourtable) b on a.receptor = b.donor
where b.donor is null
Member Avatar for tesuji

no, not that a good idea, consider:

left join (there are no null entries, because each left value has its mate in right column)
1 4
2 5
4 6

right join (if right value has no mate in left column, null will be inserted left hand side)
1 4
null 5
null 6

possibly, select looks like:
select b.donor, b.receptor from inthebelowtable a right join inthebelowtable b
on a.receptor = b.receptor where a.donor IS NOT null

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