单选题

给定供应关系SPJ(supplier_id(供应商号)，part_id(零...

Question

给定供应关系SPJ(supplier_id(供应商号)，part_id(零件号)，project_id(工程号)，num(数量))，查询某工程至少用了3家供应商（包含3家）供应的零件的平均数量，并按工程号降序排列，以下（）个sql是正确的

A. select project_id,avg(num) as 平均数量 from SPJ group by project_id having count(distinct(supplier_id)) > 2 order by project_id desc

B. select project_id,avg(num) as 平均数量 from SPJ group by project_id where count(supplier_id) > 2 order by project_id desc

C. select project_id,avg(num) as 平均数量 from SPJ group by project_id having (distinct(supplier_id)) > 2 order by project_id desc

D. select project_id,avg(num) as 平均数量 from SPJ group by project_id where supplier_id > 2 order by project_id desc