I have searched for this information both on SO and on google, but have not found any authoritative answer.

When you have an update statement like:

```
UPDATE table SET rowA = rowB, rowB = NULL ...

```

It seems that:

* ordering is not important (`UPDATE table SET rowB = NULL, rowA = rowB`)
* nonetheless, the result is that rowA takes the prev value in rowB, because it seems that UPDATE first reads the previous values, then it updates them.

I would like to know if the two above points are *true in general* for SQL, i.e. if they are *part of the SQL UPDATE semantics*, if they are in the *standard*, or if it is an implementation details (and therefore subject to change).
Thanks!

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> EDIT: Let me stress that I would like an "authoritative" answer; I already tested on a number of SQL implementation that the behaviour is indeed the one depicted here. What I need is a "proof" that this is actually in the SQL standard/spec/semantics of UPDATE, with a link to the standard or, alternatively, to a surrogate reliable source (MSDN, dev.mysql.com, Oracle or PostgreSQL docs, ...)
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