As per documentation:
MySQL Docs
Your workaround would be to create a view for each of your subqueries.
Then access those views from within your view view_credit_status
create view view_clients_credit_usage as
select client_id, sum(credits_used) as credits_used
from credit_usage
group by client_id
create view view_credit_status as
select
credit_orders.client_id,
sum(credit_orders.number_of_credits) as purchased,
ifnull(t1.credits_used,0) as used
from credit_orders
left outer join view_clients_credit_usage as t1 on t1.client_id = credit_orders.client_id
where credit_orders.payment_status='Paid'
group by credit_orders.client_id)
As the more recent MySQL documentation on view restrictions says:
Before MySQL 5.7.7, subqueries cannot be used in the FROM clause of a view.
This means, that choosing a MySQL v5.7.7 or newer or upgrading the existing MySQL instance to such a version, would remove this restriction on views completely.
However, if you have a current production MySQL version that is earlier than v5.7.7, then the removal of this restriction on views should only be one of the criteria being assessed while making a decision as to upgrade or not. Using the workaround techniques described in the other answers may be a more viable solution - at least on the shorter run.
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