Child Relationship Queries (Subqueries)
By the end of this lesson, you'll be able to:
- Query a parent record together with a list of its child records in one query
- Access the resulting child records as a nested List
- Explain the naming difference between a parent query and a child subquery
Prerequisites: "Parent Relationship Queries"
Querying children with a subquery
List<Account> accounts = [
SELECT Id, Name,
(SELECT Id, LastName FROM Contacts)
FROM Account
];
for (Account acc : accounts) {
System.debug(acc.Name + ' has ' + acc.Contacts.size() + ' contacts');
for (Contact con : acc.Contacts) {
System.debug(' - ' + con.LastName);
}
}The nested (SELECT Id, LastName FROM Contacts) is the subquery — it pulls every child Contact related to each Account. Note Contacts (plural) here, not Contact — this is exactly Module 16's "Nested Collections" idea applied to query results: each Account ends up with its own List<Contact> attached.
Why "Contacts" instead of "Contact"
A parent relationship (previous lesson) uses the object name (Account.Name) because there's exactly one parent. A child relationship uses a relationship name, almost always the plural form (Contacts, Opportunities) because there can be many children — this naming difference is a genuine, common trip-up worth remembering explicitly.
Handling an Account with zero related contacts
Account acc = [SELECT Id, Name, (SELECT Id FROM Contacts) FROM Account WHERE Id = :someAccountId];
if (acc.Contacts.isEmpty()) {
System.debug('This account has no contacts.');
}If an Account has no related Contact records, acc.Contacts still returns a valid, empty List<Contact> — never null — so isEmpty() and size() are always safe to call directly, no null-check required first.
Exercise
Write a query on Account selecting Id, Name, and a subquery of related Opportunities (Id, Name), then debug how many opportunities the first account has.
Show hint
(SELECT Id, Name FROM Opportunities)
Child Relationship Queries (Subqueries) Quiz
My Notes
Log in to keep private notes on this lesson.
Questions about this lesson
No questions yet — be the first to ask.
Log in to ask a question about this lesson.
Summary
A child relationship query (or subquery) pulls a parent record and every one of its related child records in a single query — the reverse direction from the previous lesson's parent traversal.