SQL - 如何计算列的唯一组合

Question

我不确定这是否可行,但我想计算表中唯一值的数量.我知道要计算我做的唯一folderID的数量:

select count(folderid) from folder

但我想在文件夹表中计算folderid和userid的唯一组合数.有没有办法做到这一点？

Answer 1

select count(*) from (
  select distinct folderid, userid from folder
)

Answer 2

select count(*) from (
    select folderId, userId
    from folder
    group by folderId, userId
) t

Answer 3

这将为您提供唯一的folderid和userid组合的计数:

SELECT count(*)
  FROM (
        SELECT DISTINCT
               folderid,
               userid
          FROM folder
);

希望能帮助到你...