The merge method is an Apex Data Manipulation Language (DML) operation that allows you to combine up to three records of the same sObject type into one of the records, known as the master record. The merge method deletes the other records, known as the victim records, and reparents any related records to the master record. The merge method has the following characteristics:
* Merge is supported with accounts, contacts, cases, and leads: The merge method can only be used with these four standard sObject types. You cannot use the merge method with custom objects or other standard objects.
* The merge method allows up to three records, including the master and two additional records with the same sObject type, to be merged into the master record: The merge method takes an array of sObject records as the input parameter, where the first record is the master record and the rest are the victim records. The merge method can merge up to three records of the same sObject type into the master record. If you want to merge more than three records, you need to call the merge method multiple times.
The other statements are not valid considerations when using the merge method:
* The field values on the master record are overwritten by the records being merged: This is not true. The field values on the master record are not overwritten by the victim records, unless you explicitly specify the field names and values in the master record before calling the merge method. By default, the merge method preserves the field values of the master record and discards the field values of the victim records.
* External ID fields can be used with the merge method: This is not true. The merge method does not support external ID fields as input parameters. You need to use the record IDs of the sObject records to perform the merge operation.
References: Merging Records, merge method, Prepare for Your Salesforce Platform Developer I Credential