Question

SELECT Y/X as Z
FROM (
    SELECT
    (count(county) FROM `cleanup_site_list_2011` WHERE county='kings') as X,
    (count(county) FROM `cleanup_site_list_2011` WHERE county='kings' AND people != 0) as Y
    FROM `cleanup_site_list_2011`
) as innertable

In the above example the two “SELECT …” represent nested queries of any description (X and Y will be integers). When I try to use this statement I get the error

#1054 - Unknown column 'Y' in 'field list'

How would I get MySQL to recognize the names of the columns I created? Also, once the math works out, how would I make the division result in a float? Or does it do this automatically?

Answer 1

One way to deal with this is to give the columns their ‘custom names’ in a subquery, then use those custom names in the surrounding query:

SELECT 
  Y/X as Z -- moved to outer query
FROM (
  SELECT 
    (...) as X, -- assign custom names in subquery
    (...) as Y, 
  FROM `table`
) as innertable;

---

For the second question:

mysql> select 4 / 3;
+--------+
| 4 / 3  |
+--------+
| 1.3333 |
+--------+
1 row in set (0.00 sec)

Does that answer it? (MySQL 5)

More information here.