oracle 10g sql與子句編譯錯誤

Question

編譯錯誤說「mm」和「cc」是無效標識符！

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

這個錯誤是因爲mm是子查詢分解（AKA CTE）實例的名稱，但你可以看到：

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

您還沒有宣佈mm爲連接到m實例。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

我認爲你正在試圖獲得大部分課程的指導。

你能不能用

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