oracle 10g sql子句编译错误

Question

编译错误说“mm”和“cc”是无效标识符！

with m as (
  select instructor, 
         count(*) as c 
    from class 
group by instructor),
     mm as ( 
  select max(m.c) as cc 
    from m)
select m.instructor 
  from m 
 where m.c = mm.cc;

Answer 1

错误是因为mm 是子查询分解 (AKA CTE) 实例的名称，但如您所见：

SELECT m.instructor 
 FROM m 
WHERE m.c = mm.cc;

您尚未将mm 声明为m 实例的JOIN。使用：

WITH m AS (
    SELECT instructor, 
           COUNT(*) as c 
      FROM CLASS
  GROUP BY instructor),
     mm AS ( 
    SELECT MAX(m.c) as cc 
      FROM m)
SELECT m.instructor 
  FROM m
  JOIN mm ON mm.cc = m.c

Answer 2

我想你是想找一位上课最多的老师。

你不能用吗

Select m.instructor FROM (select instructor, count(*) as c from class group by instructor order by 2 desc) m where rownum = 1