Starting the Trigger Debugger
You can start debugging a routine by using the Debug Routine button in the toolbar, after selecting a trigger from the database treeview, or by using the context menu in the database treeview.
After this, a Debug Trigger Details dialog will be shown, similar to the one below.
The Debug Trigger Details dialog
If your trigger is a trigger that can fire on multiple actions, the Fire Operation will be a drop down box in which you can select the current operation, one of the following: Insert, Update, Delete. Depending on the action, the bottom half of the screen will have Old Values and/or New Values displayed. These represent the values before and/or after the operation. For an INSERT operation, the Old Values part will not be visible, for example.
The NULL columns signals a column value to be NULL, if you enter a value, the NULL flag becomes unchecked.
By using the table or SQL Editor button at the right site of the dialog, you can select values from the table to fill the Old Values or New Values, this makes it easier to get values to start with.
After clicking OK, the normal debugger main window will be displayed and you can start debugging. There's one difference though, the Parameters and Variables debug window will contain entries for each OLD and NEW value available in the trigger.
New and Old values are available
You can evaluate/modify these OLD and NEW values just like you would with any other variable.