Question

I have three tables:

Orders:
orderid, valuedid, valuesdesc

Customers:
customerid, cutomdesc

Groups:
groupid, groupdesc

valueid – id of a customer or a group

valuesdesc – must filled with appropriate for inserted valueid description from Customers or Groups depend on what (customer or group) user selected in the client.

So, when client send an insert query for Orders it consists orderid for new order and valuedid. And on a client side I know what user selected: group or customer.

What I need: if a new row inserted in Orders, corresponding valuesdesc for valuedid from Customers or Groups inserted in valuesdesc column.

I have an idea to insert with new order record valuesdesc which would contain key – fake value to pick the right dictionary (customers or groups), but how to create that trigger I unfortunately don’t know for now.

Answer 1

First, I must say this is a very uncommon design. When you are inserting a record into Orders, you use valuesdesc to hint at the table that valueid references. But as soon as valuesdesc gets filled by the trigger, valueid essentially loses any meaning. That is, you haven’t reveal us you’ve got any other way to tell a reference to Customers from a reference to Groups. So, you could just as well drop valueid altogether and use valuesdesc to pass both the ‘dictionary’ hint and the reference within that table.

Other than that, because valueid can reference more than one table, you cannot use a foreign key constraint on it.

Anyway, in your present design you could try this:

CREATE TRIGGER Orders_UpdateValuesdesc
ON Orders
FOR INSERT
AS
UPDATE o
SET valuesdesc = COALESCE(c.customdesc, g.groupdesc)
FROM Orders o
  INNER JOIN inserted ON o.orderid = i.orderid
  LEFT JOIN Customers c ON i.valuesdesc = 'Customers'
    AND i.valueid = c.customerid
  LEFT JOIN Groups g ON i.valuesdesc = 'Groups'
    AND i.valueid = g.customerid

The strings 'Customers' and 'Groups' are meant as the dictionary specifiers. You replace them with the actual values.

The trigger uses LEFT JOIN to join both Customers and Groups, then fills Orders.valuesdesc with either customdesc or groupdesc, depending on which one is not null.