Question

我有两张桌子：

表1。 options_ethnicity，其中包含以下条目：

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，我的查询看起来像这样：

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

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

任何帮助都将不胜感激。

以下是对这篇文章的更新，其中包含一个似乎有用的查询：

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，一种处理NULL值的ANSI标准方法。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，但在COALESCE的情况下它不能移植到其他数据库。

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

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

这将获取所有NULL ethncity_id实例，但它会将计数归属于“NULL占位符”组。 IE：

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

Answer 2

您计算了一个字符串而不是右列

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

Answer 3

为什么不“反向”查询？

SELECT 
  options_ethnicity.ethnicity_name, 
  COUNT(inquiries.ethnicity_id) AS count
FROM 
  options_ethnicity
  Left Join inquiries On options_ethnicity.ethnicity_id = inquiries.ethnicity_id
GROUP BY 
  options_ethnicity.ethnicity_id

你仍然可能需要Coalesce调用，但对我来说，这个查询对你想要完成的事情更有意义。

带有GROUP BY和零条目的MySQL COUNT

3 个答案: