SQL: Update table by mapping two columns to each other

Question

I have the following two tables:

        Table A
+-------------------+
|___User___|__Value_|
|    3     |  a     |
|    4     |  b     |
|    5     |  c     |
|____6_____|__d_____|



        Table B
+-------------------+
|___User___|__Value_|
|    1     |        |
|    4     |        |
|    5     |        |
|____9_____|________|

My job is to take user from Table A (and their correspondings value) and then map it to Table B and insert those values in there. So from the above example Table B should look like this after running the script:

        Table B
+-------------------+
|___User___|__Value_|
|    1     |        |
|    4     |  b     |
|    5     |  c     |
|____9_____|________|

My question is how can I construct an SQL query that will do this for me in an efficient way, if Table A contains 300,000 + entries and Table B contains 70,000 entries?

NOTES: In Table A the User field is not unique and neither is the Value field. However in Table B, both the User and Value fields are unique and should not appear more than once. Neither are primary keys for either tables.

Answer 1

Could be this

update table_b as b 
inner join table_a as a on a.User = b.User
set b.value = a.value 

Answer 2

In real-world situations, it would be more likely that you want a predictable value, such as the greatest value for any given user. In that case you would want

update table_b as b
inner join (
   select user, max(value) from table_a
   group by user ) as a_max on a.user = b.user
set b.value = a_max.value