Duplicate column name error during union - mysql

Question

I'm using this query to count rows resulting from 2 tables using union and an alias at the end of the query but it's not working:
SELECT COUNT(*) FROM (
(SELECT * FROM bills INNER JOIN cats INNER JOIN suppliers
INNER JOIN new_cards
WHERE new_cards.`Card_Code` LIKE '%8%'
AND bills.`Sup_ID` = suppliers.`Sup_ID`
AND new_cards.`Sup_ID` = suppliers.`Sup_ID`
AND cats.`Cat_ID` = bills.`Cat_ID`
AND bills.`Cat_ID` = cats.`Cat_ID`
AND new_cards.`Bill_ID` = bills.`Bill_ID`)
UNION
(SELECT * FROM bills INNER JOIN cats INNER JOIN suppliers
INNER JOIN sold_cards WHERE
sold_cards.`Card_Code` LIKE '%8%'
AND bills.`Sup_ID` = suppliers.`Sup_ID`
AND sold_cards.`Sup_ID` = suppliers.`Sup_ID`
AND cats.`Cat_ID` = bills.`Cat_ID`
AND bills.`Cat_ID` = cats.`Cat_ID`
AND sold_cards.`Bill_ID` = bills.`Bill_ID`)
) w
the error is
Error Code: 1060
Duplicate column name 'Cat_ID'

Answer 1

Seems you have a column called Cat_ID in both cats and bills tables. When you do a select * on a join on the tables you will get duplicate column names.
Some possible solutions:
Rename the Cat_ID column in one of the tables
Explicitly name the columns in your select, i.e. instead of "*" use "bills.Cat_ID AS new_name, ...."

Answer 2

These are not variables, it's ok to join or union tables with duplicated column names. See below.
Is the error coming from the union or the join? Try running just the first sub-select, then just the second sub-select.
As a possible workaround, it looks like you could factor out the union of the two cards tables, and SELECT FROM bills JOIN cats JOIN suppliers JOIN (SELECT * from new_cards UNION SELECT * from sold_cards) cards WHERE ...
UNION merges duplicate rows (UNION ALL returns all rows from both sides). If you know you won't have duplicates, you could avoid the union altogether and select just the sum of two counts directly, ie select (select count(*) from ...joins...) + (select count(*) from ...joins...).
None of this answers why it would return an error.
I ran a quick test, which worked:
create table a (id int, a int, b int);
create table b (id int, a int, b int);
insert into a values (1,1,1), (2,2,2);
insert into b values (3,3,3);
select count(*) from ( (select * from a) union (select * from b) ) t;
+----------+
| count(*) |
+----------+
| 3 |
+----------+
1 row in set (0.00 sec)
select * from ( (select * from a) union (select * from b) ) t;
+------+------+------+
| id | a | b |
+------+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 3 | 3 |
+------+------+------+
3 rows in set (0.00 sec)
select count(*) from a inner join b where a.a = b.a and b.a = a.a;
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.00 sec)

Answer 3

Oh, I see it -- the error is coming from the nested select temp tables.
When running the query, mysql must create a temporary table for each of the two inner selects. It's those temp tables that error out, because you can't create a table with two identically named columns.
If you equated the two Cat_ID's in the join condition instead of in the WHERE, mysql should recognize that they're identical and it should work; something like:
SELECT COUNT(*) FROM (
SELECT * FROM bills
JOIN cats USING (Cat_ID)
JOIN suppliers USING (Sup_ID)
JOIN sold_cards USING (Bill_ID, Sup_ID)
UNION
SELECT * ...
)
Or like Svea said, select a specific column like cats.Cat_ID to count. (And selecting just one column will run faster than selecting all)
select * from (select * from a join b) t;
ERROR 1060 (42S21): Duplicate column name 'id'
select * from (select * from a join b using (id, a, b)) t;
Empty set (0.00 sec)