How is this NOT key preserved?

Suppose we have a table A
Its PK has 3 columns, X, Y and Z
The table is to be joined to itself:

Code:

---


SELECT * FROM
  a src INNER JOIN a dst ON src.x=dst.x AND src.y = dst.y
WHERE
  src.z = 'source' AND
  dst.z = 'dest'

---

For any given row that has a Z column of text "source"
There can only be one other row having matching X and Y columns and a Z column of dest

The whole of the PK is accounted for, and there either exists a pair of rows, or there not exists a pair of rows


But this view is not updatable according to oracle.. How can it be written so the optimizer believes that?

Thanks in advance

Use a merge instead of an update?

I did in the end.. it just irritates me that it doesnt work when it should! ;)