mysql query to summarize data

I am trying to summarize some data from two data sources. The tables are following

Table t1

Table t2

Expected data in t1 after query:

To solve this I was planning to do the following query

REPLACE INTO t1
SELECT auto_id, unique_column, SUM(value) value FROM 
(
  SELECT * FROM t1
  UNION ALL 
  SELECT * FROM t2
) GROUP BY unique_column; 

Issue with this query

SELECT auto_id, unique_column, SUM(value) value FROM 
(
  SELECT * FROM t1
  UNION ALL 
  SELECT * FROM t2
) GROUP BY unique_column; 

The above query will give us

Which is okay. In the insert query we will get the following result:

But the following query

SELECT auto_id, unique_column, SUM(value) value FROM 
(
  SELECT * FROM t2
  UNION ALL 
  SELECT * FROM t1
) GROUP BY unique_column; 

will give us

So, Insert query will product the following unexpected result

Alternative thought was, full outer join but mysql decided not to implement this. What other alternative can be done with mysql?

You were close:

REPLACE INTO t1
SELECT MAX(auto_id), unique_column, SUM(value) value FROM 
(
  SELECT * FROM t1
  UNION ALL 
  SELECT * FROM t2
) t
GROUP BY unique_column;

MAX(auto_id) will skip NULL fields, which will give us just 3 for 'd3'; for the 'd4' case, auto_id will still be NULL, but (as long as the column is declared as AUTO_INCREMENT NOT NULL), NULL will be replaced by the next number in the sequence.

EDIT for your edit: as long as you properly aggregate auto_id using MAX (or MIN), the only scenario NULL appears is if you have no prior auto_id; it is not sensitive to ordering.