select columns from another table and create a column

I have 2 tables

ads:

+------------+------------+-------------+
|   id       |  width     |   height    |
+------------+------------+-------------+
|   1        |  300       |   250       |
|   2        |  550       |   50        |
|   3        |  300       |   250       |
|   4        |  300       |   250       |
|   5        |  550       |   50        |
+------------+------------+-------------+

assigned_ads:

+------------+------------+-------------+
|   id       |  location  |   ad_id     |
+------------+------------+-------------+
|   1        |  hp1       |   2         |
|   2        |  hp2       |   3         |
|   3        |  hp3       |   5         |
|   4        |  hp4       |   1         |
|   5        |  hp5       |   4         |
+------------+------------+-------------+

I have ad_id as a foreign key...

With PHP I will be need to output the size of the ad from ad_id but this information is held in the ads table. The infor开发者_运维技巧mation I will need is width x height. I know there's a query I can use to create an additional column with this information but for the life of me I can't remember, or find it.

Any help appreciated.

SELECT a.width, a.height
    FROM assigned_ads aa
        INNER JOIN ads a
            ON aa.ad_id = a.id
    WHERE aa.id = 123 -- Your value goes here.