5.12Query by External ID

This should look completely natural now:

SELECT
    Id,
    Name,
    Phone
FROM Account
WHERE External_Customer_ID__c = 'C001'

In Mule, prefer parameterization rather than string concatenation.

Conceptually:

SELECT Id, Name
FROM Account
WHERE External_Customer_ID__c = :customerId

with:

customerId = vars.customerId