顯示百分比的計數和計數兩個布爾值

Question

我有一個學生的表格，有一些關於他們是否需要一個教程等數據。下面是一個摘錄。

| id | studentid | name | present | 
| 1 | 111  | John |  1 | 
| 2 | 222  | Kate |  0 | 
| 3 | 333  | Matt |  1 | 
| 4 | 111  | John |  0 | 
| 5 | 111  | John |  1 | 

我可以做一個計數說，當如studentid = 111和存在= 1，當studentid = 111和存在= 0 但我不能工作了這兩樣東西。

i。我可以只進行一次計數查詢而不是分別計數嗎？ 目前正與我的兩個單獨的查詢，我得到如

studentid = 111和存在= 1

| name | present | 
| John |  12 | 

studentid = 111和存在= 0

| name | absent | 
| John |  4 | 

但我會非常喜歡

| name | present | absent | 
| John |  12 |  4 | 

ii.I想看看那些已經低於75％的課程。我已經做了一些關於avg函數的閱讀，但是我不確定如何爲此實現某些功能？

Answer 1

要列出一直以等級小於75％的學生：

SELECT 
    studentid, 
    name, 
    SUM(present) AS present, 
    SUM(present = 0) AS absent 
FROM your_table 
GROUP BY studentid, name 
HAVING present < .75 * (present+absent) 

Answer 2

你可以做象下面這樣：

SELECT 
    studentid, 
    name, 
    SUM(present = 1) AS present, 
    SUM(present = 0) AS absent 
FROM your_table 
GROUP BY studentid, name 

• SQLFiddle Demo