--create the tables
create table parent ( a serial not null, z int, primary key (a));     
create table child  ( a int REFERENCES parent(a), sal char(1));

--insert some stuff for test
insert into parent values (0, 10);  
insert into parent values (0, 20);
insert into parent values (0, 30);  
insert into child values (1, "N");  
insert into child values (2, "Y");
insert into child values (3, "Y");

--Add the child columns to the parent table
alter table parent add sal char(1);

--Move the data from child to parent
update parent set sal=(select sal from child where parent.a=child.a) where 1=1;

--Check the parent table
select * from parent;
          a           z sal
          1          10 N
          2          20 Y
          3          30 Y
          
--drop the child table
drop table child;

--create the tables
create table parent ( a serial not null, z int, primary key (a));     
create table child  ( a int REFERENCES parent(a), sal char(1));

--insert some stuff for test
insert into parent values (0, 10);  
insert into parent values (0, 20);
insert into parent values (0, 30);  
insert into child values (1, "N");  
insert into child values (2, "Y");
insert into child values (3, "Y");
              
--create new parant table    
create table parent2 (a int not null, z int, sal char(1), primary key (a));
               
--Move the data from child/parent to new parent
insert into parent2 
select     parent.a, parent.z, child.sal 
from       parent
inner join child
on         parent.a=child.a;

--change to serial
alter table parent2 modify a serial not null primary key;
        
--Check the new parent table
select * from parent2;
          a           z sal
          1          10 N
          2          20 Y
          3          30 Y
          
--drop the child and old parent table
drop table child;   
drop table parent;

--rename the new parent table
rename table parent2 to parent;