ER diagramm建模：FK到多个表

Question

I hava web application with database on SQL Server 2008. I am analyzing ER model of given application.

There is a table "Table1" with columns "ContactType" and "ContactID". Based on ContactType value "ContactID" points to different tables (Table_ContactType1 or Table_ContactType2).

So, in this case we cannot define foreign key. I wonder if this is good practice? If not, should I change model somehow? Would it cause some problems later if there are no foreign key defined?

EDIT: For now it is very difficult to combine ContactType1 and ContactType2 tables into one tables. Because CotnactType1 is a table that was used earlier and it contains users who has access to the system (login name and password, email and other information). IT is a table with many fields. Unfourtunately new requirements came and new table ContactType2 as created. This table has only three columns and those are organizations names without any access to system. It would be very difficult to combine those, becuase it would affect much functionality. The simpliest thing - when contact to contacttype1 table is added then email, username, password cannot be empty etc. But also much other.

Answer 1

You can create third table that will combine ContactType1 and ContactType2 for their common columns ( Type and ID ) and then you can make FK from your Table1 to the new combined table (let's call it ContactTypeUnified )

ContactType1 and ContactType2 can both remain as is, and also have their IDs as FK to the new ContactTypeUnified table.

You only trouble here is that data manipulation on ContactType1 and ContactType2 will also have to be reflected on ContactTypeUnified

Answer 2

In general database designs without integrity are untrustworthy and need to be avoided at all costs - so yes you do need to apply the foreign key. In this case I would just combine the two contact type tables as it is unlikely that these should be separated.