duplicate with join 2 table

Hi everyone,
I have a problem when join two table. Both of them have format 2 columns is 'nid'; 'tid'
When I join them I have some unexpected data

the first table I have
nid | tid
4 | 0
5 | 5

the second table I have
nid | tid
4 | 7
5 | 5
6 | 5

The result
nid | tid
0 | 4 ( THIS IS THE ONE UNEXPECTED)
4 | 0
4 | 7
5 | 5
5 | 5
6 | 5

My code is
SELECT * FROM
(SELECT
id_city_test.nid,
id_city_test.tid
FROM
id_city_test)
AS a

UNION

SELECT * FROM
(SELECT
id_industry_test.nid ,
id_industry_test.tid
FROM
id_industry_test)
AS b

Can someone help me.
Thanks a lot
Thang