如何从辅助表中获取唯一的行数

Question

create table pro_category (id primary key, product_id int, category_id int)
insert into pro_category (1, 1, 1)
insert into pro_category (1, 1, 2)
insert into pro_category (1, 2, 1)
insert into pro_category (1, 2, 1)

如何从辅助表中获取唯一的行数（例如，在上述情况下，涉及2个产品ID，因此我希望得到2的答案。）

Answer 1

使用count(distinct)

select count(distinct product_id) from pro_category

Answer 2

SELECT COUNT(DISTINCT product_id) FROM pro_category

这将返回2