MySQL COUNT with GROUP BY and zero entries - sql

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, i.e. 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.