Full Outer Join Coalesce

fojc.sql

-- ARBITRARY COLUMN SELECTION
create table a (x int, y int, z int);
create table b (x int, y int, z int);
create table c (x int, y int, z int);
insert into a values (1, 2, 3);
insert into b values (1, null, 30);
insert into c values (1, 200, null);
select * from a;
/*
 x | y | z 
---+---+---
 1 | 2 | 3
 */
select * from b;
/*
 x | y | z  
---+---+----
 1 |   | 30
 */
select * from c;
/*
 x |  y  | z 
---+-----+---
 1 | 200 |  
 */

select
coalesce(a.x, b.x, c.x) as x,
coalesce(b.y, a.y, c.y) as y,
coalesce(c.z, b.z, a.z) as z
from a
full outer join b on a.x = b.x
full outer join c on b.x = c.x;

/*
 x | y | z  
---+---+----
 1 | 2 | 30
*/

delete from a;
delete from b;
delete from c;

-- DATA OUTSIDE INTERSECTIONS PRESERVED
insert into a values (1,2,3);
insert into b values (1,20,30);
insert into c values (1,200,300);
insert into a values (2, null, null);
insert into c values (100, null, null);

select * from a;
/*
 x | y | z
---+---+---
 1 | 2 | 3
 2 |   |
*/
select * from b;
/*
 x | y  | z  
---+----+----
 1 | 20 | 30
 */
select * from c;
/*
  x  |  y  |  z  
-----+-----+-----
   1 | 200 | 300
 100 |     |    
 */

select
coalesce(b.x, c.x, a.x) as x,
coalesce(b.y, c.y, a.y) as y,
coalesce(b.z, c.z, a.z) as z
from a
full outer join b on a.x = b.x
full outer join c on b.x = c.x;

/*
  x  | y  | z  
-----+----+----
   1 | 20 | 30
   2 |    |   
 100 |    |   
 */