how to join 2 tables into one with SQL - sql

Question

I have 2 tables, Let's name them tb1 and tb2.
I want to add all items in tb1 that dose not exist in tb2 into new rows in tb2.
At the same time I want to update existing data in tb2 with the data in tb1, I try to understand join, merge and so on but I could not understand how doing that in SQL.
For the question I will build this 2 tables and the result I try to achieve.
tb1:
| KEY | col one | col two
+------+------------+-----------
| 1 | data one | data one
| 2 | data two | change data
| 3 | data three | data three
tb2:
| KEY | col one | col two
+------+-----------+-----------
| 1 | data one | data one
| 2 | data two | old data
| 4 | data four | some data
tb2 after SQL :
We can see we add the key 3 and we change in key 2, col 2 data
| KEY | col one | col two
+------+------------+-----------
| 1 | data one | data one
| 2 | data two | change data
| 3 | data three | data three
| 4 | data four | some data

Answer 1

You can generate the results you want using union all:
select t1.key, t1.col1, t1.col2
from table1 t1
union all
select t2.key, t2.col1, t2.col2
from table2 t2
where not exists (select 1 from table1 t1 where t1.key = t2.key);
Actually changing table2 is more cumbersome -- and depends on the database you are using. One method is an insert and update:
update table2
set col1 = (select t1.col1 from table1 t1 where t1.key = t2.key),
col2 = (select t1.col2 from table1 t1 where t1.key = t2.key)
where col1 <> (select t1.col1 from table1 t1 where t1.key = t2.key) or
col2 <> (select t1.col2 from table1 t1 where t1.key = t2.key);
insert into table2 (key, col1, col2)
select t1.key, t2.key, t3.key
from table1 t1
where not exists (select 1 from table2 t2 where t2.key = t1.key);
Specific databases have many methods for simplifying this logic, including on conflict, on duplicate key update, and merge commands.