如果SQL结果为空，则输出错误

我正在运行执行查询并在一行中输出记录的powershell脚本。

如果查询未在数据库中找到记录，如何输出通知？

我试图评估$ rdr是否为$ NULL，但是整个输出为null，好像数据库中文件中没有IP地址一样：

foreach ($k in $file){


    $conn.Open()

    $sql = "SELECT dbo.sem_computer.COMPUTER_NAME,[IP_ADDR1_TEXT],(dbo.SEM_AGENT.FREE_DISK/1073741824)as 'Free Disk Space (GB)', 
    (dbo.SEM_COMPUTER.DISK_TOTAL/1073741824) as 'Total Disk Space (GB)',
    (dbo.SEM_AGENT.FREE_MEM/1073741824) as 'Free Memory (GB)', (dbo.SEM_COMPUTER.MEMORY/1073741824) as 'Total Memory (GB)', 
    dbo.SEM_COMPUTER.OPERATION_SYSTEM,NAME, SEM_AGENT.MAJOR_VERSION
    FROM dbo.sem_computer, [dbo].[V_SEM_COMPUTER], dbo.IDENTITY_MAP, dbo.SEM_CLIENT, dbo.SEM_AGENT
    WHERE [dbo].[V_SEM_COMPUTER].COMPUTER_ID = SEM_COMPUTER.COMPUTER_ID
    AND dbo.SEM_CLIENT.GROUP_ID = IDENTITY_MAP.ID
    AND dbo.SEM_CLIENT.COMPUTER_ID = dbo.SEM_COMPUTER.COMPUTER_ID
    AND dbo.SEM_COMPUTER.COMPUTER_ID = dbo.SEM_AGENT.COMPUTER_ID
    AND [IP_ADDR1_TEXT] = '$k'"

    $cmd = New-Object System.Data.SqlClient.SqlCommand($sql,$conn)


    do{
    try{
        $rdr = $cmd.ExecuteReader()

        if ($rdr -eq $Null){
            write-host $k not found in database
        }
        # Read Computer Name, Computer ID, IP address, Domain, and Operating System from the Database into a multidimensional array

        while ($rdr.read()){
                $sql_output += ,@($rdr.GetValue(0),",", $rdr.GetValue(1), ",", $rdr.GetValue(2),",",  $rdr.GetValue(3), ",", $rdr.GetValue(4), ",", $rdr.GetValue(5),",",  $rdr.GetValue(6), ",", $rdr.GetValue(7), ",", $rdr.GetValue(8))
        }

        $transactionComplete = $true
    }
    catch{
        $transactionComplete = $false
    }
    }until ($transactionComplete)



    $conn.Close()    
}

Add-Content C:\PS\NOC_IP_Address_Statistics.csv "Computer Name,IP Address,Free Disk Space (GB),Total Disk Space (GB),Free Memory (GB),Total Memory(GB),Operating System,Group,SEP Version" 
foreach ($i in $sql_output){
    Add-Content C:\PS\NOC_IP_Address_Statistics.csv "$i"
}