The lookup field is similar to the master–detail relation, as it is used to link two objects without having the same strong binding between the parent and child records.

This means that sharing is not inherited, you cannot have roll-up fields, the child record has its own owner, and if you delete the parent record, then the child is not automatically deleted, and you can look up the same object type.

Creating a lookup field is quite similar to creating the master–detail type:

The only difference is that you don't have the Access section, just the What to do if the lookup record is deleted? option. If the parent record is deleted, you can choose to blank the field or throw an error so that the user is forced to update this relation before deleting the parent.

As lookup is not a required field (unlike a master-detail field), you can decide to put it as Required.

Filters apply the same way that we saw earlier when we looked at the master–detail field type.

If the main object is a custom object, then you can ask Salesforce Support to enable the Delete this record also option, which makes the lookup close to a master–detail relation, at least regarding the deletion behavior: this is called cascading delete.

Lookups can reference the same object type (which is not allowed in master–detail relationships), creating a so-called self-relationship: the only limitation is that you cannot look up a record with itself (this is called a circular reference).