Combine identical elements of a matrix to filter informations

Hello everybody,

I have a couple of matrices with two columns: a District code and the regarding ZIP codes.

              276050166000                     41749
              276050166000                     41751
              276050166000                     47877
              276050170000                     46519
              276050170000                     46535

I would like to create sub matrices where I can group the zip codes to their regarding district codes. As it is a huge dataset, I would like to let it be done automatically (f.e. with a loop!?). Something like if I have the value i=276050166000, 276050168000,... then give me the regarding zip codes. The objective is to filter the zip codes and combine them with another dataset in order to make conclusions about every single district.

I want to have something like this, but that I do not have to calculate every district "by hand":

kreis0966=ismember(RB09(:,1),276050966000);

K0966 = RB09(kreis0966,:);

PLZ0966 = K0966(:,2);

i0966 = ismember(clearedDataInt(:,2),PLZ0966);

Passiva0966 = clearedDataInt(i0966,4);

As I am a beginner in Matlab, I have no idea how to do this and I am thankful for every help :)