Merging data from one table to another

Hello - 

I have 2 tables in my model that I am trying to 'merge' the data together.  I am looking to pull one data element from Table1 and put it in Table2.  The two tables have a relationship based on the Transaction ID (but both tables are joined to a calendar table so I cannot make this relationship active).  The tables are very large, so I do not really want to use a calculated column in the table.

            Table 1			                     Table 2	
Transaction ID	Asset		Transaction ID	  Approval Date
AAA123	        1		     AAA123	    1/1/2018
AAA123	        2		     AAA345	     2/1/2018
AAA123	        3			
AAA345	        4 			

So I am trying to get the approval dates into Table1 the most efficient way possible, without causing performance issues in my report.  (Table 1 has millions of rows of data).

Thanks for any help

Ryan