使用SQL COALESCE：
COALESCE（Count（distinct pd.numbers），0）AS“Count”

解决这个问题的唯一方法是遵循@JNevill的建议-这是一个常见的问题，解决方案是一个标准模式。此外，可能有一个标准函数会给予你月份名称，而不是数字，所以你不需要CASE语句--但是因为你没有用你正在使用的DBMS标记你的问题，所以不可能知道。
无论如何，类似以下的东西应该可以工作-您可能需要针对特定的DBMS进行调整：

with month_list(month_name) as
        select *
        from values
        ('January'),
        ('February'),
        ('March'),
        ('April'),
        ('May'),
        ('June'),
        ('July'),
        ('August'),
        ('September'),
        ('October'),
        ('November'),
        ('December')
    )
    SELECT 
    ml.month_name
    Count(distinct pd.numbers) AS "Count"       
    FROM month_limit ml
    LEFT OUTER JOIN Power_D pd on ml.month_name = TO_CHAR(pd.Date,'MMMM')
    AND EXTRACT(YEAR FROM pd.Date) = '2016' 
    AND pd.Name = 'Test' 
    GROUP BY  ml.month_name
    ;

这是基于@JNevill评论的MySQL/MariaDb的可能解决方案

SELECT m.month_name, count(distinct pd.numbers) AS "Count"
FROM 
(
    SELECT 1 AS month, 'January' as month_name
    UNION ALL SELECT 2, 'February'
    UNION ALL SELECT 3, 'March'
    UNION ALL SELECT 4, 'April'
    UNION ALL SELECT 5, 'May'
    UNION ALL SELECT 6, 'June'
    UNION ALL SELECT 7, 'July'
    UNION ALL SELECT 8, 'August'
    UNION ALL SELECT 9, 'September'
    UNION ALL SELECT 10, 'October'
    UNION ALL SELECT 11, 'November'
    UNION ALL SELECT 12, 'December'
    
) m

LEFT JOIN 

(
    SELECT *    
    FROM Power_D 
    WHERE EXTRACT(YEAR FROM Date) = '2016' 
      AND Name = 'Test' 
      
) pd ON EXTRACT(MONTH FROM pd.Date) = m.month

GROUP BY m.month_name
ORDER BY m.month