MySQL order by other tables row - mysql

Question

I've got those two tables:
Table a:
--- ID --- Name ---
Table b:
--- ID --- ID_of_a --- Date ---
Now I want to order the rows of table a by the Date (desc) of table b by the newest date.
Example:
Table a has rows with ID "1" and "2".
Table b has rows like this: {ID, ID_of_a, Date} {1, 1, "2013-06-30"}, {2, 1, "2013-07-01" }, {3, 2, "2013-07-02" }
The correct order of IDs from table a would be: 1 --- 2
Query:
SELECT DISTINCT a.ID, a.Name FROM a, b WHERE a.ID=b.ID_of_a ORDER BY b.Date desc
But this doesn't work sometimes.

Answer 1

Assuming there is a 1-1 relationship between the tables:
select a.*
from a left outer join
b
on a.id = b.id
order by b.date desc
If there are multiple rows in b for each a, then you need a group by:
select a.*
from a left outer join
b
on a.id = b.id
group by a.id
order by max(b.date) desc

Answer 2

How about
select a.name,a.id from a join (select id_of_a as id_a,max(dt) as max_dt from b group by id_of_a order by max_dt desc) e on e.id_a=a.id
Example
mysql> select * from a;
+----+------+
| id | name |
+----+------+
| 1 | a |
| 2 | b |
+----+------+
2 rows in set (0.00 sec)
mysql> select * from b;
+----+---------+------------+
| id | id_of_a | dt |
+----+---------+------------+
| 1 | 1 | 2013-06-20 |
| 2 | 2 | 2013-07-01 |
| 3 | 1 | 2013-07-02 |
+----+---------+------------+
3 rows in set (0.00 sec)
mysql> select a.name,a.id from a join (select id_of_a as id_a,max(dt) as max_dt from b group by id_of_a order by max_dt desc) e on e.id_a=a.id;
+------+----+
| name | id |
+------+----+
| a | 1 |
| b | 2 |
+------+----+
2 rows in set (0.00 sec)
Basically you first select the maximum date for each id_of_a and then join it to a
I think it can be re-written to run more efficiently