MySQL; Get information from 3 table same time - mysql

Question

People
here is my little problem.
I have three table:
a_names_1
b_names_2
c_names_3
they are same by structure. all of them has two item: name and used
Is there any QUERY to run to get and count all the 'name' that has 'used'=1 from all those three tables together.
I've tried this one, but didn't work:
(SELECT COUNT(*) 'name' from a_names_1) UNION
(SELECT COUNT(*) 'name' from a_names_2) UNION
(SELECT COUNT(*) 'name' from a_names_3) WHERE `used`=1
I'm using PHPMyAdmin for MySQL.
Any Help would be appreciated.. thanks in advance

Answer 1

This query outputs count of distinct names from all tables with used=1
select count(distinct name)
from
(
select name,used from a_names_1 where used=1
union all
select name,used from a_names_2 where used=1
union all
select name,used from a_names_3 where used=1
) t
If you need to SUM all USED for each NAME from all tables and output only with SUM of used=1 then:
select count(*) from
(
select name, SUM(used)
from
(
select name,used from a_names_1
union all
select name,used from a_names_2
union all
select name,used from a_names_3
) t
GROUP BY name
HAVING SUM(used)=1
) t1

Answer 2

select count(*) as name
from
(
select name, used from a_names_1
union
select name, used from a_names_2
union
select name, used from a_names_3) t
where t.used = 1

Answer 3

Probably this is slow, because you lose the index optimizations. What I would do is do the three queries, something like
SELECT SUM('name') AS name_sum
FROM ((SELECT COUNT(*) 'name' from a_names_1 WHERE `used`=1)
UNION (SELECT COUNT(*) 'name' from a_names_2 WHERE `used`=1));
If this doesn't work, it is probably a problem with the usage of name

Answer 4

Maybe you wanted this way:
select count(*) as cnt
from
(
select name from a_names_1 t1 where t1.used = 1
union
select name from a_names_2 t2 where t2.used = 1
union
select name from a_names_3 t3 where t3.used = 1
) t

Answer 5

The straight forward solution;
SELECT SUM(used) FROM (
SELECT used FROM a_names_1 WHERE used=1
UNION ALL
SELECT used FROM a_names_2 WHERE used=1
UNION ALL
SELECT used FROM a_names_3 WHERE used=1
) a
SQLfiddle for testing
An alternative if you have an index on used (and the only values of used are 0 or 1) is to just do the counting using the index;
SELECT SUM(used) total FROM (
SELECT SUM(used) used FROM a_names_1
UNION ALL
SELECT SUM(used) FROM a_names_2
UNION ALL
SELECT SUM(used) FROM a_names_3
) a
SQLfiddle for this example.
If you look at the query plan of the latter query, you can see it uses the indexes effectively.