MySQL中两个表的总体唯一计数

Question

I have two tables, both having column a device_id column that I want to count. For the purposes of demonstration, the schema looks like:

Table 1: 'id', 'save_val', 'device_id_major'
Table 2: 'id', 'save_val', 'location', 'device_id_team'

Table 1 could have many of the same ' device_id_major '.

I basically want to get the unique device_id's from both tables, then from that result set, get the count of unique device_id's (the same device_id can appear in both tables).

Is this possible in one query?

Answer 1

select distinct aa.device_id, count(*) 
from(select distinct device_id from table1
union all
select distinct device_id from table2) as aa
group by device_id
order by device_id

Or something like... As I don't have the schema to hand, I can't fully validate it.

Answer 2

SELECT count(DISTINCT aa.id) 
FROM (SELECT DISTINCT major_id AS id FROM `major` 
UNION ALL
SELECT DISTINCT team_id AS id FROM `team`) 
AS aa

This seems to do the trick.

Answer 3

您可以使用获取两个表的UNION的查询，然后选择唯一值。