Very often people who works with SQL Server database get the following
error when they trying to open database diagrams after restoring
database from back up:
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.
To fix this problem you should execute this T-SQL script:
EXEC sp_dbcmptlevel 'yourDB', '90';
go
ALTER AUTHORIZATION ON DATABASE::yourDB TO "yourLogin"
go
use [yourDB]
go
EXECUTE AS USER = N'dbo' REVERT
go