Question

我有一个要求，我想用where子句和group by运行SELECT查询。它返回几行，我想从中选择数量最少的行。

示例：

table t1:
host_name   application  type  
host1        app1         0
host2        app2         0
host3        app4         0
host1        app5         0
host2        app6         0
host1        app7         1
host2        app8         1

在上表中，我想获得负载最少的主机。

host1 运行app1，app5，app7。

host2 运行app2，app6，app8。

host3 仅运行app4。

所以host3负载最少。当我运行查询时，输出应为“host3”。只能选择type=0行。

我有解决方案。但它是否优化了？是否有最佳的实现方法？

SELECT host_name from (SELECT host_name, min(count) FROM (SELECT host_name, COUNT(*) as count FROM t1 where type=0 group by host_name) as Dup ORDER BY host_name);

提前致谢：）

Answer 1

试试这个，它会返回host个apps的最少select `host_name` from t1 where type = 0 group by `host_name` order by count(`host_name`) ASC limit 0,1。

    logger.info("************************Start of executeChannel() method*****************************");
    Channel channel = session.openChannel("exec");

    // below line avoids "sudo: no tty present and no askpass program" error

    ((ChannelExec) channel).setPty(true);

    // this line ensures password prompt is not displayed after sudo user
    /**
    * -S  The -S (stdin) option causes sudo to read the password from the standard input instead of the terminal device.
    * -p  The -p (prompt) option allows you to override the default password prompt and use a custom one.
    */

    String filename = file.toString().replace("[", "").replace("]", "").replace(",", "");   
    System.out.println("sudo -S -p '' "+command+" "+filename+" server2:/dir1/dir2/dir3/");

    ((ChannelExec)channel).setCommand("sudo -S -p '' "+command+" "+filename+" server2:/dir1/dir2/dir3/");

    channel.setInputStream(null);            
    ((ChannelExec)channel).setErrStream(System.err);

    InputStream in = channel.getInputStream();
    OutputStream out=channel.getOutputStream();

    BufferedReader br = new BufferedReader(new InputStreamReader(in));

    channel.connect();
    out.write((pwd+"\n").getBytes());


    byte[] tmp=new byte[1024];
    while(true){
        logger.info("start of while(true) method, is channel connected? "+channel.isConnected());

        br.readLine();
        while(br.readLine() != null){
            System.out.println("channel.isConnected() "+channel.isConnected());
            int i = in.read(tmp, 0, 1024);

            if(i<0) break;

            System.out.println("printing putty console: \n"+new String(tmp,0,i));
        }
        if (channel.isClosed()){
            System.out.println("Exit Status:- "+channel.getExitStatus());
            break;
        }

        try{
            Thread.sleep(1000);
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
    out.flush();
    br.close();
    System.out.println("DONE");

}

SELECT查询根据行数

1 个答案: