On Sat, 2007-03-17 at 12:19 -0400, Mark wrote:
> Robert Cummings wrote:
>
> > On Sat, 2007-03-17 at 09:43 -0500, Myron Turner wrote:
> >> Colin Guthrie wrote:
> >> > Philip Thompson wrote:
> >> >
> >> >> For auto increment values, you don't have to specify the id. For
> >> >> example:
> >> >>
> >> >> INSERT INTO t_users (f_name, l_name, e_mail, b_date, pic)
> >> >> VALUES ('$f_name', '$l_name', '$e_mail', '$b_date', null);
> >> >>
> >> >> Then to find the latest entry:
> >> >>
> >> >> SELECT user_id FROM t_users ORDER BY user_id DESC limit 1;
> >> >>
> >> >
> >> > This is not the cleanest way and some databases do not actually
> >> > increment auto id fields (e.g. they could fill in the blanks from
> >> > previous deletes etc.).
> >> >
> >> > Much better is to use the function in the MySQL API to get the insert
> >> > id or if you really must use SQL, just run "SELECT LAST_INSERT_ID();"
> >> > which does much the same thing.
> >> >
> >> > Col
> >> >
> >> >
> >> My own knowledge of mysql is about 5 years old and never really used.
> >> But I was recently asked to do something that required some mysql
> >> (noting too much, fortunately for me), so I've been doing some reading
> >> and am interested in questions that come up on the list.
> >>
> >> An earlier post called attention to a concurrency problem. Wouldn't
> >> getting the last inserted ID from LAST_INSERT_ID()
> >> suffer from the same limitations as any of the other solutions which do
> >> a select to get the last ID?
> >
> > No, it's a MySQL specific feature that is atomic with the insert and so
> > you are guaranteed that the the returned ID is the exact automatic ID
> > associated with the most recent INSERT for the connection handle.
> > Unfortunately auto increment is MySQL specific and so it isn't
> > transferrable to other database engines.
>
> The problem is if the ID returned is not somehow linked to the transaction
> which inserted the row. If you have an insert followed by a select to
> retrieve the ID, in the interrum, another user may be created by another
> process and you will retrieve the wrong ID.
I don't believe this is an issue for MySQL. The last inserted ID isn't
stored for the database table, it's stored for the connection to the
database. As such there's no race condition unless the developer does
something silly, like issue another insert over the same connection
before requesting the previous last inserted id. Just to be sure though
the following is from the MySQL online documentation:
::::
For LAST_INSERT_ID(), the most recently generated ID is maintained in
the server on a per-connection basis. It is not changed by another
client. It is not even changed if you update another AUTO_INCREMENT
column with a non-magic value (that is, a value that is not NULL and not
0). Using LAST_INSERT_ID() and AUTO_INCREMENT columns simultaneously
from multiple clients is perfectly valid. Each client will receive the
last inserted ID for the last statement that client executed.
::::
http://dev.mysql.com/doc/refman/5.0/en/getting-unique-id.html
Cheers,
Rob.
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