can we replace full join with union of left and right join? why not?

Question

can we replace full join with union of left and right join? if no,why?

Answer 1

'YES' if T1 and T2 are sets (no duplicated rows), otherwise the answer is 'NO'.

create table t1 (i int);
create table t2 (i int);

insert into t1 values (1);
insert into t1 values (2);
insert into t1 values (2);

insert into t2 values (3);

---

FULL JOIN

select * from t1 full join t2 on t1.i=t2.i 
order by 1,2

---

1   (null)
2   2
2   2
(null)  3

---

UNION

select * from t1 left join  t2 on t1.i=t2.i
union
select * from t1 right join t2 on t1.i=t2.i
order by 1,2

---

1   (null)
2   2
(null)  3  

---

UNION ALL

select * from t1 left join  t2 on t1.i=t2.i
union all
select * from t1 right join t2 on t1.i=t2.i
order by 1,2

---

1   (null)
2   2
2   2
2   2
2   2
(null)  3

Answer 2

The above explanation really helps. Just one add on the outputs are valid only if we have below value inserted in t2 "insert into t2 values (2);"