There will be situation while creating the database diagram, you got the below error message.
Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.
Above error is self explanatory where it is stating that the “set the database owner to valid login” To fix the issue please follow below one of the solution.
Execute the below script by mentioning the database name.
ALTER AUTHORIZATION ON DATABASE::MentionDatabaseName TO sa GO
OR, you can change the owner by executing the below query.
EXEC sp_changedbowner 'sa'