问题
I am using django orm and I am trying to get all the values of a column, but only if a different column is unique with respect to it. Its hard to explain, so here is an example:
q | a | 1
w | s | 2
e | a | 3
q | a | 4
w | s | 5
e | a | 6
I would like to get all the values that are in column 2 but if they also have the same value in column 1 don't take duplicates. So in this case I want to get [a,s,a]. (column 3 only serves to show why these duplicates don't get merged in the first place).
What I tried:
I tried grouping by values of columns 1 and 2 and taking the distinct value of it, thus I would end up with:
q | a
w | s ---> which is actually given as [(q,a),(w,s),(e,a)]
e | a
with the code: queryset.values(col1,col2).distinct()
but I only want the second column so I then added .values(col2). The problem with that is the distinct gets applied to the results of the second values as well as the first, so instead of getting [a,s,a] I get [a,s].
I also tried using .defer() so queryset.values(col1,col2).distinct().defer(col1) but apparently you can't use .defer() after using .values().
I can't find a solution to this online and am getting nowhere for a while now, any help would be greatly appreciated!
回答1:
If you are using PostgreSQL as your database, try this:
queryset.order_by('col1', 'col2').distinct('col1', 'col2').values('col2')
I haven't had a chance to test it, but it should find results where the combination of col1 and col2 are distinct, and return the col2 values.
来源：https://stackoverflow.com/questions/52905220/django-queryset-get-distinct-column-values-with-respect-to-other-column