Question

我使用MySQL，并且我有一个代码可以从每个类别中获取最后6篇文章：

select a.*
from article a
where a.created >= coalesce((select a2.created
   from article a2
   where a2.category = a.category
   order by a2.created desc
   limit 5, 1
  ), a.created
 );

现在，我还需要从另一个表中获取每篇文章的总视图。怎么做？这不起作用：

select a.*, Count(view.*) as CountViews
from article a
where a.created >= coalesce((select a2.created
   from article a2
   where a2.category = a.category
   order by a2.created desc
   limit 5, 1
  ), a.created
 ) left join view on a.id = view.post_id;

按类别打印文章的示例：https://nedvigimostmsk.ru/

Answer 1

由于Gordon现在似乎已离线，因此我将在此处发布我的编辑内容作为另一个答案。这应该给您所需的订单。

    select a.*,
           (select count(*)
            from views v
            where a.id = v.post_id
           ) as num_views
    from article a
    where a.created >= coalesce((select a2.created
                                 from article a2
                                 where a2.category = a.category
                                 order by a2.created desc
                                 limit 5, 1
                                ), a.created
                               )
ORDER BY a.created DESC;

Answer 2

我可能建议使用子查询：

[CmdletBinding()]
param()

Trace-VstsEnteringInvocation $MyInvocation

$appdirectory = get-vstsinput -Name appdirectory
$webappname = get-vstsinput -Name webappname
$ResourceGroupName = get-vstsinput -Name ResourceGroupName

#write-host $appdirectory

$location="East US"
    try {
    # Get inputs.
    # Get publishing profile for the web app
    $xml = [xml](Get-AzureRmWebAppPublishingProfile -Name $webappname `
    -ResourceGroupName $ResourceGroupName `
    -OutputFile null)

    # Extracts connection information from publishing profile
    $username = $xml.SelectNodes("//publishProfile[@publishMethod=`"FTP`"]/@userName").value
    $password = $xml.SelectNodes("//publishProfile[@publishMethod=`"FTP`"]/@userPWD").value
    $url = $xml.SelectNodes("//publishProfile[@publishMethod=`"FTP`"]/@publishUrl").value

    Write-Output "$username"
    Write-Output "$password"
    Write-Output "$url"
    #Write-Output "$localpath"

    # Upload files recursively
    Set-Location $appdirectory
    $webclient = New-Object -TypeName System.Net.WebClient
    $webclient.Credentials = New-Object System.Net.NetworkCredential($username,$password)
    $files = Get-ChildItem -Path $appdirectory -Recurse | Where-Object{!($_.PSIsContainer)}
    foreach ($file in $files)
    {
        $relativepath = (Resolve-Path -Path $file.FullName -Relative).Replace(".\", "").Replace('\', '/')
        $uri = New-Object System.Uri("$url/$relativepath")
        "Uploading to " + $uri.AbsoluteUri
        $webclient.UploadFile($uri, $file.FullName)
    }
    $webclient.Dispose()

    } finally {
        Trace-VstsLeavingInvocation $MyInvocation
    }

查询的问题是您使用的{ "id": "LongStringofNumbers", "name": "NameOfTask", "friendlyName": "FriendlyNameOfTask", "description": "Build Task that will upload a file or folder to a destination in Azure using Published Profile Credentials.", "helpMarkDown": "", "category": "Build", "visibility": [ "Build" ], "runsOn": [ "Agent", "DeploymentGroup" ], "author": "ME", "version": { "Major": 0, "Minor": 0, "Patch": 16 }, "instanceNameFormat": "Uploads File Using Published Profile Credentials", "groups": [ { "name": "advanced", "displayName": "Advanced", "isExpanded": true } ], "inputs": [ { "name": "appdirectory", "type": "filePath", "label": "Source Path", "defaultValue": "", "required": true, "helpMarkDown": "Location of file(s) for uploading to Azure." }, { "name": "webappname", "type": "string", "label": "Azure Webapp name", "defaultValue": "", "required": true, "helpMarkDown": "Azure App name; I.E. - 900014campuslinkapi." }, { "name": "ResourceGroupName", "type": "string", "label": "Azure Resource Group name", "defaultValue": "", "required": true, "helpMarkDown": "Azure Resource Group Name I.E. - 900014-prod." } ], "execution": { "PowerShell3": { "target": "powershell.ps1", "platforms": [ "windows" ], "argumentFormat": "", "workingDirectory": "$(currentDirectory)" } } }没有select a.*, (select count(*) from views v where a.id = v.post_id ) as num_views from article a where a.created >= coalesce((select a2.created from article a2 where a2.category = a.category order by a2.created desc limit 5, 1 ), a.created );。要么返回错误（在最新版本的MySQL中使用默认设置），要么返回一个神秘的行。您可以使用COUNT()来解决此问题，但是相关的子查询通常具有更好的性能-尤其是使用正确的索引。

Answer 3

作为Gordon的替代方案，您还可以对嵌套查询进行联接，该查询从views表开始计数和分组

select a.*, v.cnt as total_views
from article a
inner join 
    (
        select post_id, count(*) as cnt from views group by post_id
    ) v on a .id = v.post_id
where a.created >= coalesce((select a2.created
   from article a2
   where a2.category = a.category
   order by a2.created desc
   limit 5, 1
  ), a.created
 );

Answer 4

我会尝试

... )left join view on a.id = view.post_id 
GROUP BY a.*

此外，在您的SELECT语句中，使用它代替COUNT（view。*）AS CountViews

COUNT(view.post_id) AS CountViews

一般来说，最好列出要返回的特定列，而不是仅使用SELECT a.*

SQL：如何从按类别排序的多篇文章中获得观看次数计数？

4 个答案: