Master-Detail Relationship in Salesforce

Master-Detail relationship in Salesforce means a parent-child relationship between any two objects in which the parent object controls some certain behaviour of detail object i.e. child object.

Some Features of Master-Detail Relationship

It is a strongly coupled Relationship.
Parent object record is required on child object record.
The standard objects cannot be child objects.
Whenever you delete the parent object record then the child object record which is related to the parent object will automatically delete. It is known as Cascade deletion of records.
The roll-Up summary field is only available for the parent object.
You can create a maximum of 2 Master-Detail relationships for any object.
The Owner field on the detail object is not available but it is automatically set to the owner of the associate master object record.
Custom object as detail object in a master-detail relationship cannot have sharing rules, manual sharing, or queues, as these require the Owner field.
The detailed object records follow the security and sharing setting of the master object record.
This relationship can manage between ‘standard - custom objects’ and ‘Custom - Custom objects’.But not with ‘standard - standard objects’ because standard objects never be a child or detail object. But custom objects can be parent or child objects.
When we undeleting master object record then automatically child object record associated with that master object record gets undeleted.
Master-Detail relationship field is not required on child objects.

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Important Note

You can create your Master-Detail Relationship Field on child objects only.
Firstly you have to select your child and parent object then proceed further.
You can not create a Master-Detail Relationship Field on any object when there will be any record already present on a child object.

How to create Master-Detail Relationship

Step - 1: Go to the ‘object manager’.

Step - 2: Select your child object on which you want to create your Master-Detail Relationship. Here I select ‘Company’ as my child object.

Step - 3: Click on the ‘Fields and Relationships’.

Step - 4: Click on the ‘New’ button to create the Master-Detail Relationship field.

Step - 5: Now select the ‘Master-Detail Relationship’ data type.

Step - 6: Click on ‘Next’.

Step - 7: Select Parent Object in the ‘Related to’ field. Here you can select any standard or Custom Object as a parent object according to your need. Here I select the ‘Account’ object from the drop-down menu as the parent object.

Step - 8: Click on the ‘Next' button.

Step - 9: Fill in the details -

Field Label - you can give any name here according to your requirement. Here I gave ‘Account’ name. Mostly we give object names for master-detail relationships.
Field Name - It automatically takes a Field Label name.
Description - Describe your field details here. It is an optional field.
Help Text - Give information about this field. It is also an optional field
Child Relationship Name - ‘Companies’ Here your child object name automatically gets populated.
Sharing Setting - Here you can give read/write or read access for this Master-detail relationship field.
Allow Reparenting - If you checked this checkbox then it will allow for reparenting your master-detail relationship field. It is by default unchecked by Salesforce itself.
Auto Add to Custom report type - It is by default checked the checkbox. It allows this field for the custom report type.

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Step -10: Here you can Establish Field level security for reference fields. If the field is a master-detail relationship then it automatically assigns visibility to all profiles.