Select a row based on the value of another row - mysql

Question

App goal
Send messages from retailers to registered customers mobile via GCM
DB architecture
I have a customers table and a related customers_realtions table with the following fields: id, customerID, retailerID, isBlocked
Required outcome
A customer can register for a specific retailer, or for a wildcard (all of them).
In case one registers for all the retailers, he has an option to block a specific retailer from sending future messages, effectively creating a blacklist.
DB values for each status
When a customer registers for a single retailer retailerID is assigned with the retailer ID.
When a customer register for all of the retailers retailerID equals 1.
When a customer blocks a retailer there are two options:
a. if he registered to this specific retailer before the isBlocked field is updated to 1 (true)
b. if he registered to all retailers before a new row is created for this retailer and isBlocked is set to 1 (true)
The challenge
When sending the message the SELECT query should include the customers that has a retailerID of 1 and does not have the sending retailerID when isBlocked equals 1.
For example, in this situation
id customerID retailID isBlocked
129 46 111 1
128 46 1 0
I don't want the customer to be selected even if the retailerID is 111
My attempt
SELECT * FROM customers_relations
WHERE
(retailID=111
OR
(retailID=1
AND
(SELECT isBlocked FROM `customers_relations` WHERE customerID=46 AND retailID=111)=0))
AND
NOT isBlocked
Question
While this is working for a single customer for whom I know the ID in advance, I am struggling to figure a way of writing a similar query for multiple customers.

Answer 1

I think of this as an aggregation query. You want to look through all the rows that are not blocked for a customer and determine whether retailer 111 is available or all retailers are available:
SELECT customerId
FROM customers_relations cr
WHERE isBlocked = false
GROUP BY customerId
HAVING MAX(retailId = 111) > 0 OR
MAX(retailId = 1) > 0;
I notice that your question actually says that a new row is created in customer_relations when someone is blocked. The above assumes there is one row. To handle the case when a block on any row would cause a block, then:
SELECT customerId
FROM customers_relations cr
GROUP BY customerId
HAVING (MAX(retailId = 111) > 0 OR
MAX(retailId = 1) > 0
) AND
MAX(retailId = 111 AND isblocked = true) = 0;