From section 7.2.6.2 of the mysql manual:
The column is not specified explicitly in an UPDATE statement
and some other column changes value. (Note that an UPDATE that
sets a column to the value it already has will not cause the
TIMESTAMP column to be updated, because if you set a column
to its current value, MySQL ignores the update for efficiency.)
> On Fri, 2 Apr 1999 09:48:05 -0800, Mike Wexler wrote:
>
> > INSERT INTO myTable(value, id) VALUES ("my value", "myKey");
> > SELECT stamp FROM myTable WHERE id="myKey";
> >
> > # Time consuming operation here
> >
> > UPDATE myTable SET value="my value" WHERE id="myKey";
> > SELECT stamp FROM myTable WHERE id="myKey";
>
> Why do you call this a bug? You are clearly updating the record, even
> though you're putting in a value that happens to be the same as the one
> that was there. There is no reason for the SQL server to look at
> "value" in the record to satisfy your request. It simply finds records
> matching the WHERE part and places "my value" into the field "value".
>
> If you want the time stamp updated only on change, you need to do a
> SELECT, identify the records where you want to change, and then change
> only those.
>
>
> -Sincerely, Fred
>
> (Frederik Lindberg, Infectious Diseases, WashU, St. Louis, MO, USA)
>
>
>
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