How to select count of two distinct columns?

Question

我想计算从这个SQL返回的不同COMPONENT_ID和PACKAGE_ID的总数。

SELECT DISTINCT COMPONENT_ID, PACKAGE_ID FROM BILL;

Answer 1

You can use:

select count(distinct (COMPONENT_ID || PACKAGE_ID))
  from BILL;

Provided || is the string concatenation operator.

Answer 2

select count(*)
from
(
    select distinct component_id, package_id
    from yourtable
) as distinctquery

Answer 3

SELECT COUNT(*) FROM (
    SELECT DISTINCT component_id FROM bill
    UNION SELECT DISTINCT package_id FROM bill
)