Issue updating in MySQL where the count of selected rows is 1 - mysql

Question

I am having a problem updating a table with a simple update statement.
The data looks like this
+----------------------------------------------------------------------+
| id | contractor_id | domain | primary_domain |
_______________________________________________________________________|
| 1 | 50 | foo.com | 1 |
|----------------------------------------------------------------------|
| 2 | 50 | foo.net | 0 |
|----------------------------------------------------------------------|
| 3 | 100 | widget.com | 0 |
+----------------------------------------------------------------------+
What I am trying to do is update only contractors that have 1 domain and that domain is set to 0 ... So in this case it would ignore contractor 50 and set contractor 100's primary_domain to 1
This is what I've come up with, but it's failing and I am not sure why -- Can someone explain my short-coming? How do I achieve the intended result?
UPDATE domains_test SET primary_domain = '1'
WHERE(
SELECT primary_domain
FROM domains_test
WHERE primary_domain = 0
HAVING domain = 1);
EDIT
I have even tried HAVING count(domain) = 1);
The response is always
You can't specify target table 'domains_test' for update in FROM clause

Answer 1

To correctly get the contractor_id values related to just one domain you can use the following query:
SELECT contractor_id
FROM domains_test
GROUP BY contractor_id
HAVING COUNT(DISTINCT domain) = 1;
Output:
contractor_id
--------------
100
So, the UPDATE can be written as:
UPDATE domains_test
SET primary_domain = 1
WHERE primary_domain = 0 AND
contractor_id IN (SELECT contractor_id
FROM (
SELECT contractor_id
FROM domains_test
GROUP BY contractor_id
HAVING COUNT(DISTINCT domain) = 1) AS t);
The nesting in the subquery is necessary due to the error:
you can't specify target table 'domains_test' for update in FROM clause
This error appears in case contractor_id is obtained directly from domains_test in the subquery.
Demo here

Answer 2

SQL DEMO
UPDATE domains_test AS d
LEFT JOIN
(SELECT contractor_id
FROM domains_test
GROUP BY contractor_id
HAVING COUNT(*) = 1) as f
ON d.contractor_id = f.contractor_id
SET d.primary_domain = 1
WHERE d.primary_domain = 0
AND f.contractor_id IS NOT NULL;

Answer 3

Your logic is off-colour: You want to update domains where the COUNT of a subquery is a certain value; in this case, where a count of contractors with only 1 domain and that domain is of 0 value.
you currently do not count anything in your SQL. Instead use this:
UPDATE domains_test SET primary_domain = 1
WHERE (
SELECT COUNT(*) FROM domains_test
GROUP BY (contractor_id)
) = 1
AND primary_domain = 0
What this does: First it searches for groups of contractors where the contractor_id occurs only once in the count, and then for each on of these it checks the value of primary_domain, updating as appropriate when both conditionals are true.