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Lookup vs master-detail (and why it matters)

Lookup vs master-detail

Salesforce has two main ways to relate objects, and choosing between them is a classic exam and interview question.

Lookup

A loose link, like a nullable foreign key.

• The child can exist without a parent (unless you make the field required)
• Delete the parent and, by default, the lookup on the child is cleared
• The child has its own owner and sharing
• Example: Contact.AccountId (a Contact can exist without an Account)

Master-detail

A tight parent-child bond.

• The child (detail) can't exist without its parent (master)
• Delete the master and its details are deleted too (cascade delete)
• The detail has no owner: security and sharing are inherited from the master
• The master can have roll-up summary fields (COUNT, SUM, MIN, MAX of the children)
• Example: OpportunityLineItem under Opportunity

	Lookup	Master-detail
Parent required	Optional	Always
Cascade delete	No (clears field)	Yes
Child owner	Own	Inherited
Roll-up summaries	No (needs Flow/Apex)	Yes
Max per object	40	2

In code: relationship names

Relationships get a relationship name used for traversal in SOQL:

// child → parent: dot notation
SELECT Name, Account.Name FROM Contact

// parent → child: subquery using the child relationship name
SELECT Name, (SELECT LastName FROM Contacts) FROM Account

For custom relationships the name ends in __r: Quest__c.Hero__r.Name.