带有GROUP BY和零条目的MySQL COUNT

Question

我有2张桌子:

表1. options_ethnicity包含以下条目:

ethnicity_id ethnicity_name  
1 White  
2 Hispanic  
3 African/American  

表2. inquiries包含以下条目:

inquiry_id ethnicity_id  
1 1  
2 1  
3 1  
4 2  
5 2  

我想生成一个表格,显示按种族划分的查询次数.到目前为止,我的查询如下所示:

SELECT options_ethnicity.ethnicity_name, COUNT('inquiries.ethnicity_id') AS count
FROM (inquiries 
    LEFT JOIN options_ethnicity ON
    options_ethnicity.ethnicity_id = inquiries.ethnicity_id)  
GROUP BY options_ethnicity.ethnicity_id

该查询给出了正确的答案,但没有非洲/美国的列有0个结果.

White 3  
Hispanic 2

如果我用右连接替换LEFT JOIN,我会获得所有3个种族名称,但非洲/美国人的数量是错误的.

White 3  
Hispanic 2  
African/American 1

任何帮助,将不胜感激.

这是对这篇文章的更新,看起来似乎是一个有效的查询:

SELECT 
    options_ethnicity.ethnicity_name, 
    COALESCE(COUNT(inquiries.ethnicity_id), 0) AS count 
FROM options_ethnicity LEFT JOIN inquiries ON inquiries.ethnicity_id = options_ethnicity.ethnicity_id 
GROUP BY options_ethnicity.ethnicity_id 

UNION ALL

SELECT 
    'NULL Placeholder' AS ethnicity_name, 
    COUNT(inquiries.inquiry_id) AS count 
FROM inquiries 
WHERE inquiries.ethnicity_id IS NULL 

Answer 1

因为您正在使用LEFT JOIN,所以对LEFT JOIN中定义的表的引用可以为null.这意味着您需要将此NULL值转换为零(在本例中):

   SELECT oe.ethnicity_name, 
          COALESCE(COUNT(i.ethnicity_id), 0) AS count
     FROM OPTIONS_ETHNICITY oe
LEFT JOIN INQUIRIES i ON i.ethnicity_id = oe.ethnicity_id
 GROUP BY oe.ethnicity_id

此示例使用COALESCE,ANSI标准处理NULL值的方法.它将返回第一个非null值,但如果找不到任何值,则返回null. IFNULL是MySQL的有效替代品,但它不能移植到其他数据库,而COALESCE则是.

在真实数据库表中,在查询表中有一些条目,其中ethnicity_id为NULL,即没有记录种族.有关如何计算这些空值以便显示它们的任何想法？

我想我理解你所面临的问题:

   SELECT oe.ethnicity_name, 
          COALESCE(COUNT(i.ethnicity_id), 0) AS count
     FROM (SELECT t.ethnicity_name,
                  t.ethnicity_id
             FROM OPTIONS_ETHNICITY t
           UNION ALL
           SELECT 'NULL placeholder' AS ethnicity_name,
                  NULL AS ethnicity_id) oe
LEFT JOIN INQUIRIES i ON i.ethnicity_id = oe.ethnicity_id
 GROUP BY oe.ethnicity_id

这将拾取所有NULL ethncity_id实例,但它会将计数归因于"NULL占位符"组.IE:

ethnicity_name   |  COUNT
------------------------
White            |  3  
Hispanic         |  2
NULL placeholder |  ?