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

我想要的是总数中的注册人数，这是COUNT(enrollment_id)的用途。然后，我想计算结果列中的NULL值的数量，以获得unfinalised_enrollments的列。

底部的WHERE子句是否会影响使用结果列的其他SELECT语句？如果是这样，我如何制作它，以便我只能计算结果列中NULL值的数量，并将其显示在名为unfinalised_enrollment的列中。

提前感谢。

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

这个应该完成你想要的：

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

您可以使用：

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null的行总数

COUNT和WHERE语句查询

3 个答案: