I understand that an UPDATE is done using "old" values of the variables
involved so that we can write
T(n + 1) = U(T(n))
where T(n) and T(n + 1) are the values of the table before and after
update U.
Thanks for all who pointed this out.
Just as a matter of interest, I want to point out that the example you
have given (below) does not _prove_ this. It can imply two situations.
1. The UPDATE query is done using the old values - the actual case.
2. The ordre of assignment statements is _undefined_! If this is the case, it is possible for the statements to take
placein the order 1, 3, 2 (not 1, 2, 3) which also gives c1, c2, c3 to be 9, 5, 9 respectively. Becaue initially, the
firstrow reads,
c1 = c2 = c3 = 1
after first assignment
c1 = 10 - c2 = 9
if third one takes place after this
c3 = 10 - c2 = 9
and finally
c2 = 5
On Sun, 29 Apr 2001, Ossie J. H. Moore wrote:
> While I'm not sure specificly which order they will be determined
> in, it has no effect on what the value of "c1" will be. The value
> of "c1" will be 10 minus the value of "c2" where "c2" equals the
> value it was before the update occurred. For exmple...
>
> 1. Assume you create the following table...
>
> create table temp
> (
> c1 int2
> , c2 int2
> , c3 int2
> );
>
> 2. Insert the following row...
>
> insert into temp values (1,1,1);
>
> 3. Execute the following update command...
>
> update temp
> set c1 = (10-c2)
> , c2 = 5
> , c3 = (10-c2);
>
> 4. Execute the following select...
>
> select * from temp;
>
> 5. Observe the following output...
>
> c1 c2 c3
> == == ==
> 9 5 9
>
> You will note that both c1 and c3 equal 10 - 1. Neither equals
> 10-5. To make the value 10 - {the value after the update},
> you would need to set the value to c1/c3 equal to the value of
> 10 - {the expression used to assign value to c2}. In the
> example above, your update statement would be...
>
> update temp
> set c1 = ( 10 - (5) )
> , c2 = (5)
> , c3 = ( 10 - (5) );
>
>
> In article <Pine.LNX.4.21.0104281846010.233-100000@presario>,
> anuradha@gnu.org wrote:
>
> >
> > If I have a query
> >
> > UPDATE tablename SET c1 = 10 - c2, c2 = 4 where ...
> >
> > will the two assignments be evaluated from left to right?
> >
> > Thanks in advance.
> >
> > Anuradha
> >
> >
