Comparing two tables in MySQL - mysql

Question

I have a master table and a temp table that look something like:
things_temp
+----+--------+--------------+
| id | number | current_time |
+----+--------+--------------+
| 1 | 456 | 9/16/2013 |
| 2 | 123 | 9/16/2013 |
+----+--------+--------------+
things_master
+----+--------+--------------+-----+
| id | number | last_updated | old |
+----+--------+--------------+-----+
| 1 | 456 | 9/15/2013 | 0 |
| 2 | 234 | 9/15/2013 | 0 |
| 3 | 888 | 8/14/2012 | 1 |
+----+--------+--------------+-----+
I need to iterate through the things_temp table and, if there exists the same number in things_master AND old == 0, update the last_updated to the current_time.
Otherwise, if both conditions above are not satisfied, simply add the record from things_temp to things_master with last_updated as current_time and old = 0.
Now, I could easily get the count of things_temp and check each one individually. But there are something like 40,000 records in each table so I think that may be a bad idea.
I've been looking around and there are things like UNION ALL, LEFT JOIN, INNER JOIN that all seem like they may be a part of the solution, but I'm a bit lost.
Is there a better way to accomplish my task without iterating through each record of things_temp and searching through things_master?

Answer 1

You might be able to do this in one statement by abusing replace into, but it's probably clearer to do it in two steps. Other databases support merge which is designed for this sort of thing.
start transaction;
-- update any matching numbers with the data from thing_temp
update
things_master m
inner join
things_temp t
on m.number = t.number
set
m.last_updated = t.`current_time`
where
m.old = 0;
-- add any missing numbers
insert into
things_master (number, last_updated, old)
Select
number, `current_time`, 0
From
things_temp t
Where
not exists (
select
'x'
from
things_master m
where
t.number = m.number and
m.old = 0
);
commit transaction;