Group by does not provide accurate grouping

Attempting to create outputs that match the following screenshot:

When I attempt the following query:

SELECT t.amount, d.DOMAIN_NAME,td.month_number
FROM transaction t 
JOIN transaction_date td ON t.trans_date_key = td.trans_date_key
JOIN domain d ON t.domain_key = d.domain_key
WHERE td.month_number =7
ORDER BY amount DESC;

I get the output of:

When I implement this query:

SELECT t.amount, d.DOMAIN_NAME,td.month_number
FROM transaction t 
JOIN transaction_date td ON t.trans_date_key = td.trans_date_key
JOIN domain d ON t.domain_key = d.domain_key
WHERE td.month_number =7
GROUP BY domain_name
ORDER BY amount DESC;

I get the output of:

Why is my grouping only performing accurately on a few of the domain names, but not others?

You are not using GROUP BY correctly. You would need to use an aggregate function to sum the amounts. On the other hand, all non-aggregated columns should be listed in the GROUP BY clause.

Consider:

SELECT SUM(t.amount) total_amount, d.domain_name, td.month_number
FROM transaction t 
INNER JOIN transaction_date td ON t.trans_date_key = td.trans_date_key
INNER JOIN domain d ON t.domain_key = d.domain_key
WHERE td.month_number = 7
GROUP BY d.domain_name, td.month_number
ORDER BY total_amount DESC;

What happens with the way you used GROUP BY is that MySQL actually picks a random record out of those that have the same domain_name. On most other RDBMS (and in non-ancient versions of MySQL), this would have generated a syntax error.