Exception: the database does not have a valid owner. Therefore, the database relational diagram support object cannot be installed.

When you restore the database backed up from another machine to your machine and click the database relationship diagram of this database in SSMs, the following prompt is displayed:

The database does not have a valid owner, so you cannot install the database relational diagram support object. To continue, first set the database owner to a valid login name using the "file" page or the alter authorization statement in the "database properties" dialog box, and then add a database relational diagram support object.

The database is determined to have a database relationship diagram. The following two methods are provided: the first method is difficult to use after several attempts, and the second method is successful. Specifically, run the following command in SSMs:

Alter authorization on Database: mydbname to sa

Modify mydbname to the actual database name, and set the owner to sa.

For detailed parameters of alter authorization, refer to the following address:
Http://msdn2.microsoft.com/zh-cn/library/ms187359.aspx