带有GROUP BY和零条目的MySQL COUNT

Question

I have 2 tables:

Table 1. options_ethnicity with the following entries:

ethnicity_id ethnicity_name  
1 White  
2 Hispanic  
3 African/American  

Table 2. inquiries with the following entries:

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

I want to generate a table that shows the number of inquires by ethnicity. My query so far looks like this:

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

The query gives the correct answer but there is no column for African/American which has 0 results.

White 3  
Hispanic 2

If I replace the LEFT JOIN with a RIGHT JOIN, I get all 3 ethnicity names, but the count for African/American is wrong.

White 3  
Hispanic 2  
African/American 1

Any help would be appreciated.

Here's an update to this post with what seems to be a working query:

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

Because you're using a LEFT JOIN, references to the table defined in the LEFT JOIN can be null. Which means you need to convert this NULL value to zero (in this case):

   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

This example uses COALESCE , an ANSI standard means of handling NULL values. It will return the first non-null value, but if none can be found it will return null. IFNULL is a valid alternative on MySQL, but it is not portable to other databases while COALESCE is.

---

In the real database table, there are some entries in the inquires table where the ethnicity_id is NULL, ie the ethnicity was not recorded. Any idea on how to get these null values to be counted so that they can be shown?

I think I understand the issue you're facing:

   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

This will pickup all the NULL ethncity_id instances, but it will attribute the counting to the "NULL placeholder" group. IE:

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

Answer 2

You counted a string instead of the right column

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

Why don't you "reverse" your query?

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

You still might need a Coalesce call, but to me, this query makes more sense for what you're trying to accomplish.