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8.2

Selecting distinct records based on a max value of one of those records

Asked by HarpuaFSB in PHP and Databases

Tags: PHP and MySQL

I have a "sessions" table in MySQL that tracks user activity.

This table is constructed as follows:
sessionid
sessionuserid
sessionip
sessiondatetime
sessionpage

Whenever there is activity on my site, a record gets inserted or updated in that table appropriately.

Users can view "Who's Online" and see the current activity on the site.

Problem is that if a user logs in from two different browsers, two entries are made into the session table, different sessionids but the same userid so select distinct will not work and I want to avoid doubling up queries by just selecting the distinct user ID in the table and then querying the database for the other infomation as I loop through that recordset.

I'd like to have it all in one query.

Is there any way to select records from this table based on a max value of the sessiondatetime? That way when I select the records, I'm only going to get the most recently updated session row.

Kind of like this even though it doesn't work:
SELECT x FROM y WHERE MAX(z) = z

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             //Current select statement
$select = "SELECT DISTINCT u.userid, ";
$select .= "u.displayname, ";
$select .= "u.gender, ";
$select .= "DATE_FORMAT(FROM_DAYS(TO_DAYS(NOW())-TO_DAYS(u.birthdate)), '%Y') + 0 AS age, ";
$select .= "u.postalcode, ";
$select .= "u.country, ";
$select .= "\"-\" AS distance, ";
$select .= "u.privatecity, ";
$select .= "u.blockwinks, ";
$select .= "u.hideonlinestatus, ";
$select .= "u.lastlogindatetime, ";
$select .= "u.moderator, ";
$select .= "u.administrator, ";
$select .= "s.sessionpage ";
$from = "FROM users u, ";
$from .= "sessions s ";
$where = "WHERE u.userid = s.sessionuserid AND ";
$where .= "TIMESTAMPDIFF(SECOND, s.sessiondatetime, now()) <= 300 ";
 
Sample REC A:
sessionid = SESSIONX
sessionuserid = 1
sessionip = x.x.x.x
sessiondatetime = 2008-06-30 09:08:48
sessionpage = home
 
REC B:
sessionid = SESSIONY
sessionuserid = 1
sessionip = x.x.x.x
sessiondatetime = 2008-06-30 09:08:40
sessionpage = whosonline
 
Using the select statement above will return two records, I just want to return REC A.
 
Keep in mind that there will be more than two records in this table so a select distinct doesn't work.
           
         

20091021-EE-VQP-81 / EE_QW_2_20070628

Selecting distinct records based on a max value of one of those records

Asked by HarpuaFSB in PHP and Databases

Tags: PHP and MySQL

About this solution