使用AVG函數確定SQL查詢中的百分比

我想知道多少百分比的記錄具有給定值，其中百分比定義爲與值匹配的記錄數除以記錄總數。即如果有100條記錄，其中10條對於student_id具有空值而20條具有999999的值，那麼percentage_999999應該是20％。我可以使用AVG功能來確定嗎？使用AVG函數確定SQL查詢中的百分比

選項1：

SELECT year, college_name, 
     sum(case when student_id IN ('999999999') then 1 else 0 end) as count_id_999999999, 
     count_id_999999999/total_id as percent_id_999999999, 
     sum(case when student_id IS NULL then 1 else 0 end) as count_id_NULL, 
     count_id_NULL/total_id as percent_id_NULL 
     count(*) as total_id 
FROM enrolment_data ed 
GROUP BY year, college_name 
ORDER BY year, college_name;

選項2：

SELECT year, college_name, 
     sum(case when student_id IN ('999999999') then 1 else 0 end) as count_id_999999999, 
     avg(case when student_id IN ('999999999') then 1.0 else 0 end) as percent_id_999999999, 
     sum(case when student_id IS NULL then 1 else 0 end) as count_id_NULL, 
     avg(case when student_id IS NULL then 1.0 else 0 end) as percent_id_NULL 
     count(*) as total_id 
FROM enrolment_data ed 
GROUP BY year, college_name 
ORDER BY year, college_name;

來源

2014-10-28 b00kgrrl

您正在使用哪些DBMS？ Postgres的？甲骨文？ – 2014-10-28 14:04:33

Oracle。我不確定它是否重要，但我正在使用Oracle SQL Developer。 – b00kgrrl 2014-10-28 14:09:54

Option1不起作用，因爲您無法在同一個選擇列表中引用列別名。 Option2是可以的，除了「percent_id_NULL」之後的逗號 – Multisync 2014-10-28 14:21:47

我創建與100條記錄，20 999999999s，10個空值，和70個的1的類似的表。這爲我工作在SQL Server上：

select count(*), StudentID 
from ScratchTbl 
group by StudentID; 

(No column name) StudentID 
10     NULL 
70     1 
20     999999999 

select avg(case when StudentID = '999999999' then 1.0 else 0.0 end) as 'pct_9s', 
     sum(case when StudentID = '999999999' then 1 else 0 end) as 'count_9s', 
     avg(case when StudentID is null then 1.0 else 0.0 end) as 'pct_null', 
     sum(case when StudentID is null then 1 else 0 end) as 'count_null' 
from ScratchTbl 

pct_9s  count_9s pct_null count_null 
0.200000 20   0.100000 10

我有你在GROUP BY子句中使用，可以爲您創造問題的感覺，也許是使用WHERE子句選擇特定的年/大學（和擺脫一行一行），看看你是否能得到你期望的結果。

來源

2014-10-28 14:21:54

使用AVG函數確定SQL查詢中的百分比

回答

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