sql - Get rows where two values are unique to each other -

the values of interest ein , registration number reg.
there lots of records each value.

what want know pair of values appears each other (or blank ein).

id     ein      reg 12     321      124 13     321      125 14     322      168 15     322      168 16     323      171 17     323      171 18     323 

so in above example, i'd want select 322 , 168, every time appear, appear together, want select 323 , 171, since never appear value, non-value. 100% of records have ein, smaller portion have registration id.

any suggestions on how query this?

one of many possible ways:

select distinct ein, reg   test t  reg not null ,    not exists (    select 1 test t1    t1.ein = t.ein , t1.reg <> t.reg    or    t1.reg = t.reg , t1.ein <> t.ein); 

ein | reg ----+----- 323 | 171 322 | 168 

-> sqlfiddle.

• i assuming ein not null, reg can null, since wrote:

100% of records have ein, smaller portion have registration id.

• null never qualifies operator <>, (323, 171) not excluded.

• i exclude rows reg is null result, seems intention. (323, null) not included.