sql如何合并同一张表里的字段并去重计数？

Question

例：表名为order
ID 买家ID 卖家ID
1 1129 1038
2 2309 3346
3 1038 3346
4 5574 1208
5 1208 2309

一个人可能既是买家也是卖家，我想计算整体的用户数，请问SQL怎么写？
select count(distinct 买家ID + 卖家ID) from order; ?

Answer 1

SELECT 买家ID FROM order
UNION
SELECT 卖家ID FROM order

Answer 2

select count( distinct( ids) ) from (select 买家ID ids from order union select 卖家ID from order);