Understanding Foreign Key and Parent-Child Relationship SOQL Queries

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; }
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