sql - COUNT 和 WHERE 语句查询

Question

目前我有这个

SELECT
    COUNT (enrollment_id) AS enrollments,
    COUNT (result) AS 
    MAX (result) AS highest_result,
    AVG (result) AS average_result,
    MIN (period_code) AS earliest_enrollment    
FROM 
    enrollment
WHERE
result IS NULL  

我想要的是总的注册人数，这COUNT(enrollment_id)是为了什么。然后我想计算NULL结果列中的值的数量以获得unfinalised_enrollments.

WHERE底部的子句会影响SELECT使用结果列的其他语句吗？如果是这样，我该怎么做才能只NULL计算结果列中值的数量并将其显示在名为unfinalised_enrollment.

提前致谢。

Answer 1

您可以使用SUM代替COUNT：

SELECT
    COUNT (enrollment_id) AS enrollments,
    SUM (CASE WHEN result IS NULL THEN 1 ELSE 0 END) AS null_results,
    MAX (result) AS highest_result,
    AVG (result) AS average_result,
    MIN (period_code) AS earliest_enrollment    
FROM enrollment    

Answer 2

这应该完成您正在寻找的东西：

SELECT
    COUNT (enrollment_id) AS enrollments,
    ( SELECT COUNT (result) FROM enrollment WHERE result IS NULL ) AS unfinalised_enrollment,
    MAX (result) AS highest_result,
    AVG (result) AS average_result,
    MIN (period_code) AS earliest_enrollment    
FROM 
    enrollment;

Answer 3

你可以使用：

SELECT
    COUNT (enrollment_id) AS enrollments,
    Count(*) - Count(result) AS null_results,
    MAX (result) AS highest_result,
    AVG (result) AS average_result,
    MIN (period_code) AS earliest_enrollment    
FROM enrollment   

Count(*)将计算总行数，Count(result)enter code here将计算结果不为空的总行数