Question

我有类似的问题：distinct values as new columns & count

但不是只有3个值（在上面的情况下：驱动程序），我有大约100万，所以我不能在我的代码中列出所有这些。我怎么能在SQLite中做到这一点？

所以我想要像i = 1到length（DISTINCT（driver））那样重复下面的代码：

    SELECT model
    , COUNT(model) as drives
    , SUM(distance) as distance
    , SUM(CASE WHEN driver=DISTINCT(driver)[i] THEN 1 ELSE 0 END) AS DISTINCT(driver)[i]
    FROM new_table
    GROUP BY model;

Answer 1

SQLite没有动态SQL机制。您必须从数据库中读取所有可能的驱动程序列表，并为程序中的每个值构建一个单独的SUM（CASE ...）列的查询。

但是大量的列效率很低，当它大于2000时，它无论如何都不会起作用。最好单独返回每个矩阵条目：

@MethodDescription(value = "Return the Server Sent Event")
@GET
@Consumes(SseFeature.SERVER_SENT_EVENTS)
@Produces(SseFeature.SERVER_SENT_EVENTS)
public EventOutput getServerSentEvents() {
    id = id + 1;
    idn = sensorA + " " + id;

    final EventOutput eventOutput = new EventOutput();
    Client client = ClientBuilder.newBuilder().register(SseFeature.class).build();
    WebTarget target = client.target("http://" + LocalNetworkIP + "/" + sensorA);
    final EventInput eventInput = target.request().get(EventInput.class);

    new Thread(new Runnable() {
        @Override
        public synchronized void run() {
            try {
                while (!eventInput.isClosed()) {
                    // Thread.sleep(500);
                    InboundEvent inboundEvent = eventInput.read();

                    if (inboundEvent == null) {
                        break; // connection has been closed
                    }
                    try {
                        // handleevent
                        // inboundEvent.readData(String.class);
                        System.out.println(inboundEvent.readData(String.class));
                        OutboundEvent.Builder eventBuilder = new OutboundEvent.Builder();
                        eventBuilder.name(inboundEvent.getName());
                        eventBuilder.data(inboundEvent.readData(String.class));
                        OutboundEvent event = eventBuilder.build();
                        eventOutput.write(event);
                    } catch (IOException e) {
                        try { //extra
                            eventOutput.close(); //extra
                            eventInput.close(); //extra
                        } catch (IOException ioClose) { //extra
                            throw new RuntimeException("Error when closing the event output internal.", ioClose); //extra
                        }    //extra  
                        throw new RuntimeException("Error when writing or reading the event.", e);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }

            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    if (!eventOutput.isClosed()) { //extra
                        eventOutput.close();       //extra
                    }                              //extra
                    if (!eventInput.isClosed()) {  //extra
                        eventInput.close();
                    }                              //extra
                } catch (IOException ioClose) {
                    throw new RuntimeException("Error when closing the event output.", ioClose);
                }
            }
        }
    }).start();
    return eventOutput;
}

}

使列的每个不同值成为新列，并在SQLite中计数

1 个答案: