Some things are meant to be learnt the hard way. And that is how I learnt about today’s gotcha.
Overview of [sysdiagrams]
I have been working on an ERD (Entity Relationship Diagram) recently and used the Database Diagram feature in SSMS for this purpose. When you try to create a diagram for the first time in a database, a message box asks you if you would like to create diagramming objects.
On clicking Yes, a system table by the name of [sysdiagrams] is created in the same database that you are creating the diagram in. The diagrams are stored in this table. SSMS shows the diagram in the Database Diagrams node of the Object Explorer tree view.
I was creating my diagram in a test environment. At some point I had to refresh all data from the production environment. The easiest way for me to do a full refresh is to use the Import and Export Wizard. The wizard can be launched either via SSMS context menu or in the Business Intelligence Developer Studio as an SSIS project. As usual in the case of quick data refreshes, I selected all tables using the top-left Source checkbox in the wizard, and chose the options to delete and reinsert all rows with identity values.
When the wizard ran successfully, my database diagram at the destination test system was missing!
Upon some research and trials I found that that if the diagramming capabilities in the source and destination servers are enabled, the wizard includes the [sysdiagrams] table automatically in the list of tables to refresh. As you can see, there are no other system tables in the wizard except the [sysdiagrams] table so it is easy to miss it in a long list.
So in my case, all data in the destination [sysdiagrams] table was deleted. There were no diagrams at the source so nothing was imported for this table. This outcome would have been the same with the drop and recreate option too because the destination table would have been recreated.
One needs to be careful while using the Import and Export Wizard. Uncheck this table in the selection list to preserve the diagrams at destination.