Skip to main content

delete all the values with in a group keeping only the topmost entry in each group in mysql

sometimes you're caught up in a situation where you have multiple entries against an id and you want to keep only single entry against each.

Table structure would be something like this:

Table `temp`

Entries:
user_id tstamp source
100 4/26/2015 app
100 4/26/2014 web
100 4/25/2015 app
101 4/26/2015 app
100 4/23/2015 app
101 4/20/2015 web
100 4/1/2015 app
200 4/26/2015 app
200 4/2/2015 app

Desired Output:
user_id tstamp source
100 4/26/2015 app
100 4/26/2014 web
101 4/26/2015 app
200 4/26/2015 app


Now, I want to delete the rows where source is `app` and keep the entry with max tstamp only with in a group.
Here's the sample query to achieve the same:

DELETE tmp FROM `temp` tmp join (select user_id, max(tstamp) as ts from `temp` where user_id in (select user_id from `temp` where source = 'app' group by user_id having count(user_id)>2) and source = 'app' group by user_id )t on tmp.user_id = t.user_id and tmp.tstamp != t.ts and tmp.source = 'app'


Exercise for you: Replace that inner query with join and check the performance benefits ;)

Comments

Popular posts from this blog

VBA MAcro to generate "table of contents with hyperlinks" automatically in a ppt

VBA MAcro to generate "table of contents with hyperlinks" automatically in a ppt: Function TableOfContent(count As Integer) 'count is the no. of slides in ppt Dim var As Integer Dim i As Integer, scount As Integer Dim strSel As String, strTitle As String, strb As String, strtemp As String, str As String Dim arr() As String Dim index As Integer, indexcount As Integer, slidecount As Integer Dim summary As Slide Dim para As Integer Dim slideOrder() As Integer 'To generate the Table of contents slide ReDim slideOrder(count - 2) 'Collect all the IDs of the selected slides For i = 1 To count - 2 slideOrder(i) = i + 2 Next 'Iterate over the slides in Index order slidecount = UBound(slideOrder) For scount = 1 To slidecount If ActivePresentation.Slides(slideOrder(scount)).Shapes.HasTitle Then 'Build up the ToC Text strTitle = ActivePresentation.Slides(slideOrder(scount)).Shapes("UseCase").TextFrame.TextRange.Text + ": &qu

Filter packets through jpcap

//Open an interface with openDevice(NetworkInterface intrface, int snaplen, boolean promics, int to_ms) JpcapCaptor captor=JpcapCaptor.openDevice(devices[index], 65535, false, 20); captor.setFilter("tcp && src port 6000", true); It'll filter all the TCP packets with source port number 6000. So, all we need to do is pass the parameters and use logical and, or operators.

redis server went away

It feels really awkward to see " redis: uncaught exception: Redis server went away " when it was working and suddenly this message bumps up. I faced the same situation while accessing php redis but I guess it's more generic one, more related to redis. So, here are possible solutions that I am able to find and highlighting the one which really did work for me. 1. /usr/sbin/setsebool httpd_can_network_connect=1 By default, SELinux does not allow Apache to make socket connections. So, enable the connection. 2. Try to stop the redis server and restart it again. either of these two should work: a) redis-cli b) redis server 3. There might be issues with switch, try to use different switch.