The SELECT statement of a SOQL query can be any valid SOQL statement, including foreign key and parent-child record joins. If foreign key joins are included, the resulting sObjects can be referenced using normal field notation. For example:
System.debug([SELECT Account.Name FROM Contact
WHERE FirstName = 'Caroline'].Account.Name);
Additionally, parent-child relationships in sObjects act as SOQL queries as well. For example:
for (Account a : [SELECT Id, Name, (SELECT LastName FROM Contacts) FROM Account WHERE Name = 'Acme']) { Contact[] cons = a.Contacts; } //The following example also works because we limit to only 1 contact for (Account a : [SELECT Id, Name, (SELECT LastName FROM Contacts LIMIT 1) FROM Account WHERE Name = 'testAgg']) { Contact c = a.Contacts; }