mysql - Referencing alias for calculation after UNION - mysql

Question

Ok, here's the query (pseudo-query):
SELECT *, (t1.field + t2.field) as 'result', (t1.field * t2.field) as result2 from((select as t1 limit 1) UNION ALL (select as t2 limit 1))
I need both rows returned, then do the math on the two fields into the result aliases. I know it's not graceful, but I have to kludge two queries together (the first is the union, and the second is the math)
So, how do I reference and use those two inner aliases? The inner aliases aren't accessible to the outer select.
I have a suspicion there's an obvious solution here that my brain is missing.

Answer 1

When you union two statements together your result is a single resultset. What you'll build:
FROM
(
(SELECT f1, f2 FROM table1 LIMIT 1)
UNION
(SELECT g1, g2 FROM table2 LIMIT 1)
) derived_table_1
This will give you a single result set named derived_table_ with two fields named f1 and f2 respectively. There will be two rows, one from your first SELECT statement and another from your second. The table aliases that you assigned inside your UNION query are no longer referencable. They exist only within their own SELECT statements.
If you have a relationship between Table1 and Table2 then you want a JOIN here:
SELECT
t1.f1 + t2.g1 as result1,
t1.f2 + t2.g2 as result2,
FROM
table1 as t1
INNER JOIN table2 as t2 ON
t1.f1 = t2.g1
If, instead no relationship exists, then you are probably looking for you original, and kludgy, union with a SUM in the SELECT:
SELECT
sum(derived_table_1.f1) as result,
sum(derived_table_1.f2) as result2
FROM
(
(SELECT f1, f2 FROM table1 LIMIT 1)
UNION
(SELECT g1, g2 FROM table2 LIMIT 1)
) derived_table_1
Editted to add a SQLFIDDLE with the last example: http://sqlfiddle.com/#!2/c8707/10

Answer 2

The column names or aliases for the result of a UNION are always determined by the first query. The column names or aliases defined in the subsequent queries of the union are ignored.
Demo:
mysql> create table foo ( a int, b int, c int );
mysql> insert into foo values (1,2,3);
mysql> create table bar (x int, y int, z int);
mysql> insert into bar values (4,5,6);
mysql> select a, b, c from (select a, b, c from foo union select x, y, z from bar) as t;
+------+------+------+
| a | b | c |
+------+------+------+
| 1 | 2 | 3 |
| 4 | 5 | 6 |
+------+------+------+
mysql> select x from (select a, b, c from foo union select x, y, z from bar) as t;
ERROR 1054 (42S22): Unknown column 'x' in 'field list'