Some times when you click on the Database Diagram node in object explorer in SQL
Server Management Studio,you get 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."
Solution for this error message: Run below script
EXEC sp_dbcmptlevel 'DBName', '90';
go
ALTER AUTHORIZATION ON DATABASE::DBName TO "LoginName"
go
use [yourDB]
go
EXECUTE AS USER = N'dbo' REVERT
go
Change DBName with your DataBase Name and LoginName with your User Name(Id).