Another Issue with Join

Hello everybody,

I'm trying to merge two datasets. One calculated, and one retrieved from data, given by:

    A=dataset(Output(:,2),Output(:,1),'VarNames',{'x' 'v'});
    B=dataset(datarest(:,x),datarest(:,v),'VarNames',{'x' 'v'});

|which results in something in the form of: 

 A =   x    v
        1    5
        2    7
        4    8
B =   x    v
        2    6
        3    7

In this, I want to use the x as keys, and merge it to get a dataset, which states v for every x. For this i'm using

C=join(A,B,'key',{'x' ' v' },'Type','outer','MergeKeys',true);

And the result is

C =     x    v
        1    5     
        2    6
        2    7
        3    7
        4    8

Where I want it to be:

C =       x    v
          1    5         
          2    6
          3    7
          4    8

Thus specifying that if there are values with the same key present in A and B, I want the value of B to be in the matrix of C. I hope you understand the issue, and can help me with the answer.

Thanks in advance!

Answer 1

[Tom Lane]

Your desired result has v=6 for x=2, but A and B provide different values for v in that case. One thing you could do is

join(A,B,'key','x','type','outer','mergekeys',true)

but this would leave you with one output v variable for each input, and you'd have to reconcile how you wanted to choose from one or the other.

[Roy Veldhuizen]

Thank you for your answer! I used this method, removing values that i did not wanted to return in the C matrix beforehand. This did the job, but from your answer I conclude it is not possible to use the join function with a ' preference' for a certain dataset?

[Tom Lane]

Play with this, and see if it gets you anywhere:

[C,IA,IB] = join(A,B,'key','x','type','outer','mergekeys',true,'leftvars','v','rightvars',{})

C.v(IB>0) = B.v(IB(IB>0))

I take v from the A dataset, but then I overwrite values based on the B dataset. I'm not sure if this is exactly what you want, but maybe you can work with it.