Hi Peter, thanks.
But it does not seem to work in my environment. Probably somewhere a bug on my side. Hartwig Am 07.05.2013 um 19:55 schrieb Peter Aronson: > At least for SQLite it appears to be. From the Update doc page > (http://www.sqlite.org/lang_update.html): > > "The modifications made to each row affected by an UPDATE statement are > determined by the list of assignments following the SET keyword. Each > assignment > specifies a column name to the left of the equals sign and a scalar > expression > to the right. For each affected row, the named columns are set to the values > found by evaluating the corresponding scalar expressions. If a single > column-name appears more than once in the list of assignment expressions, all > but the rightmost occurrence is ignored. Columns that do not appear in the > list > of assignments are left unmodified. The scalar expressions may refer to > columns > of the row being updated. In this case all scalar expressions are evaluated > before any assignments are made." > > And in fact I rely on it behaving this way for in one place in my SQLite code > and it seems to be working correctly. > > Peter > > ----- Original Message ---- >> From: skywind mailing lists <[email protected]> >> To: General Discussion of SQLite Database <[email protected]> >> Sent: Tue, May 7, 2013 10:46:42 AM >> Subject: Re: [sqlite] Update statement >> >> Hi, >> >> my question is: is it guaranteed that it works? >> >> Regards, >> Hartwig >> >> Am 07.05.2013 um 03:24 schrieb James K. Lowden: >> >>> On Mon, 6 May 2013 23:53:40 +0100 >>> Simon Slavin <[email protected]> wrote: >>> >>>>> How do I create this kind of update statement? >>>>> >>>>> UPDATE T SET a=0.5*(a+b), b=0.5*(b-a); >>>>> >>>>> The RHS should always be used with the values of a and b before the >>>>> assignment. >>>>> >>>>> I think that the result of this kind of statement is undefined, or? >>>> >>>> No need to worry, it will work the way you want it to work: >>>> >>>> The row is read. >>>> The new values are calculated. >>>> The new values are written to the database. >>> ... >>>> That was a terrible description. >>> >>> Actually that's not a bad approximation of what happens. Here's a >>> simpler example: >>> >>> sqlite> create table t(a int, b int); >>> sqlite> insert into t values (1,2); >>> sqlite> select * from t; >>> a b >>> ---------- ---------- >>> 1 2 >>> sqlite> update t set a=b, b=a; -- Et Voila! >>> sqlite> select * from t; >>> a b >>> ---------- ---------- >>> 2 1 >>> >>> There is no "RHS". The syntax and semantics of SQL are its own; they >>> cannot be extrapolated from other languages. >> >> I know but everybody knows what I meant, or? And its a quite brief > description. >> >>> >>> --jkl >>> _______________________________________________ >>> sqlite-users mailing list >>> [email protected] >>> http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users >> >> _______________________________________________ >> sqlite-users mailing list >> [email protected] >> http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users >> > _______________________________________________ > sqlite-users mailing list > [email protected] > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list [email protected] http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
