create database test3
go
use test3
create table tree (id int not null primary key, ParentID int, TargetID int)
create table cat1 (id int not null primary key, name varchar(max))
create table cat2 (id int not null primary key, name varchar(max))
go
create trigger fk_target_ins on tree instead of insert
as
	declare @insID int
	select @insID=TargetID from inserted
	if(not exists(select * from cat1 where id=@insID)
		and not exists(select * from cat2 where id=@insID))
		raiserror  50001  'cant Ins tree'
	else insert tree select * from inserted
go
create trigger fk_target_upd on tree instead of update
as
	declare @insID int
	declare @ParentID int
	declare @TargetID int
	select @insID=TargetID, @ParentID=ParentID, @TargetID=TargetID from inserted
	if(not exists(select * from cat1 where id=@insID)
		and not exists(select * from cat2 where id=@insID))
		raiserror  50001  'cant Upd tree'
	else update tree set
			ParentID=@ParentID,
			TargetID=@TargetID
		where id=@insID
go
INSERT [dbo].[cat2] ([id], [name]) VALUES ( 10 , N'qwe')
INSERT [dbo].[cat2] ([id], [name]) VALUES ( 20 , N'asd')
INSERT [dbo].[cat1] ([id], [name]) VALUES ( 100 , N'zxc')
INSERT [dbo].[cat1] ([id], [name]) VALUES ( 110 , N'vbn')
INSERT [dbo].[tree] ([id], [ParentID], [TargetID]) VALUES ( 1 , NULL,  10 )
INSERT [dbo].[tree] ([id], [ParentID], [TargetID]) VALUES ( 2 , NULL,  20 )
INSERT [dbo].[tree] ([id], [ParentID], [TargetID]) VALUES ( 3 , NULL,  100 )
INSERT [dbo].[tree] ([id], [ParentID], [TargetID]) VALUES ( 4 , NULL,  110 )