Postgres 14

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select version();

version

PostgreSQL 14.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-15), 64-bit

SELECT 1

create table test( sender   text 
                 , receiver text
                 ) ;

insert into test(sender, receiver)
     ( values ('A', 'B')
            , ('B', 'A')
            , ('A', 'C')
            , ('D', 'A') 
      ); 

select * 
  from test; 

CREATE TABLE

INSERT 0 4

sender
receiver

A
B

B
A

A
C

D
A

SELECT 4

select string_agg(res, ' ' order by res) 
  from ( 
        select distinct 
               case when sender = 'A'
                    then receiver 
                    else sender 
                end res
           from test
          where 'A'  in (sender, receiver)
      ) sq;   

string_agg

B C D

SELECT 1