What is a nested query in SOQL?

It is a SELECT inside another SELECT. In the SELECT list it returns child records (parent-to-child); in the WHERE clause with IN or NOT IN it only filters the outer rows (semi-join or anti-join).

Can SOQL join two unrelated objects like SQL?

No. Salesforce relationship query limitations state that relationship queries are not SQL joins, so the objects need a lookup or master-detail field between them. Otherwise run two queries and match them in Apex with a Map.

What is the child relationship name for Account and Contact?

It is Contacts, the plural of the child object, so the subquery reads FROM Contacts. Account also has Cases and Opportunities as child relationship names.

Does a subquery count as a separate SOQL query?

No. In our test, seven SELECT statements, several with subqueries, used exactly 7 queries against the governor limit.

Why do I get "Didn't understand relationship" in a SOQL query?

You used an object or field API name where SOQL expects a relationship name: FROM Contact instead of FROM Contacts, or Invoice__c. instead of Invoice__r. in a dot path.

SOQL Relationship Query: Parent, Child and Subqueries

A SOQL relationship query is one SELECT statement that also reads fields from records linked to it by a lookup or master-detail field. Think of it as walking along the lookup: up from a contact to its account, or down from an account to every contact under it, without leaving the query.

If you write Apex, build an LWC data source or answer “show me each account with its contacts”, this is how you get related data in one round trip instead of a query per record. We ran every example below in our developer org on API 67.0, so you will be able to pick the right direction, find the relationship name for any object and read the results in Apex.

This Tutorial Covers:

Two directions, two syntaxes

Every relationship has a parent side and a child side. Contact has a lookup (AccountId) to Account, so Account is the parent. Our custom pair works the same way: Invoice_Line_Item__c has a lookup to Invoice__c. The lookup or master-detail field is what makes the walk possible.