mysql - How to select the latest record of each hour in a day -


i want select latest record of each hour in given date, based on datetime column 'reading_on'. have executed below query 

hourly_max =   inverterreading                .where("date(reading_on) = ? , imei = ?", date.today, "770000000000126")                .group("hour(reading_on)")                .having("max(hour(reading_on))")  hourly_max.group_by(&:id).each { |k,v| puts v.last.reading_on }

in above query not getting required result. proper way select latest record of each hour in day. below table structure

enter image description here

select     hour(a.reading_on) hr, max(a.id),a.reading_on     inverterreadings left join    inverterreadings b on         year(a.reading_on)=year(b.reading_on)         , month(a.reading_on)=month(b.reading_on)         , day(a.reading_on)=day(b.reading_on)         , hour(a.reading_on)=hour(b.reading_on) ,      a.reading_on < b.reading_on      b.reading_on null group a.reading_on;

demo : http://sqlfiddle.com/#!2/49a69/14


-

Comments

Post a Comment

Popular posts from this blog

how to proxy from https to http with lighttpd -

i have internal site (192.168.2.1) accessible http on different server run public web-server using lighttpd. can make internal site accessible outside work follows $http["host"] == "internal.example.com" { ... proxy.server = ( "" => ( "internal" => ( "host" => "192.168.2.1", "port" => 8000 ) ) } this works, use https outside world. question is, how can proxy going https http ? i've tried this: $server["socket"] == ":443" { $http["host"] == "internal.example.com" { ... proxy.server = ( "" => ( "internal" => ( "host" => "http://192.168.2.1", "port" => 8000 ) ) } } but doesn't seem wor...
Read more

android - Automated my builds -

i have multiple app , android project want automated in build , release sequence. have ant script build , create apk of app. want have 1 script or ant file start build sequence app. maybe don't konw search, can't find easy documentation on ant. the point of use jenkins ant (or maven, i'm not sure of difference between them now) automated build sequence when people commit on project. any int appreciated maven dependencies (over ant) don't need pass around jars. recommend maven on ant day. syntax , overall process easier understand, @ end of both build tools. if able to, more recommend upgrading gradle. easy create different build types , create them ./gradlew assemble here project jake wharton can see power of gradle. there release , debug build, can make lot more if like. https://github.com/jakewharton/u2020 here android docs gradle http://tools.android.com/tech-docs/new-build-system/user-guide . if can't find looking here, check out main grad...
Read more

python - Flask migration error -

i've got application build on flask , wanted create new migration today. when run $python manage.py db upgrade i got message raise util.commanderror('only single head supported. ' alembic.util.commanderror: single head supported. script directory has multiple heads (due branching), must resolved manually editing revision files form linear sequence. run alembic branches see divergence(s). so run command $alembic branches no config file 'alembic.ini' found, or file has no '[alembic]' section any clue on about? the error messages coming alembic, use command form alembic <command> , integrating flask coming flask-migrate, need use form python manage.py db branches . to resolve multiple branches, make 1 of branches point down other branch upgrade graph straight line. see alembic's docs on branches: http://alembic.readthedocs.org/en/latest/branches.html
Read more