Parent Relationship Queries

List<Contact> contacts = [
    SELECT Id, LastName, Account.Name, Account.Industry
    FROM Contact
];

for (Contact con : contacts) {
    System.debug(con.LastName + ' works at ' + con.Account.Name);
}

1. List<Contact> contacts = [ — opens the query.
2. SELECT Id, LastName, Account.Name, Account.Industry — this is Module 18's "Relationships in Apex" lesson from the query side: Account.Name and Account.Industry pull the parent Account's fields into the same query result.
3. FROM Contact — the object being queried.
4. ]; — closes the query.
5. (blank line)
6. for (Contact con : contacts) { — loops over the results.
7. System.debug(con.LastName + ' works at ' + con.Account.Name); — no second query needed to fetch the parent's fields separately.
8. } — closes the loop.

List<Contact> contacts = [
    SELECT Id, LastName, Account.Owner.Name
    FROM Contact
];

1. List<Contact> contacts = [ — opens the query.
2. SELECT Id, LastName, Account.Owner.Name — Account.Owner.Name reaches two levels up: from Contact to its parent Account, and from that Account to its owning User's Name.
3. FROM Contact — the object being queried.
4. ]; — closes the query. Salesforce supports traversing up to 5 relationship levels deep in a single SOQL query this way.

// DON'T DO THIS — a query inside a loop
for (Contact con : contacts) {
    Account acc = [SELECT Name FROM Account WHERE Id = :con.AccountId]; // a query per contact!
}

1. // DON'T DO THIS — a query inside a loop — the label says it all.
2. for (Contact con : contacts) { — loops over the contacts already in memory.
3. Account acc = [SELECT Name FROM Account WHERE Id = :con.AccountId]; — a separate query runs for every single contact.
4. } — closes the loop.