如何编写一个SQL查询，该查询将从每分钟更新一次（并使用当前时间）的数据库中获取每小时的记录

Question

I'm working on a java program that reads from a database that is keeping various measurements of a lake minute by minute. I need to write a query that will list each variables by most recent time then skip 1 full hour and display the record at that new time, and repeat that process so I can retrieve the values hourly for the last 48 hours and store them as an array on the java side. The biggest thing is it needs to be real time. (The DB records 3120 records each hour)

StreamID is that of the sensor, MeasurementID is given to a single measurement, MeasurementValue is the what the sensor returns, and MeasurementTime is using the datetime data type YYYY-MM-DD HH:MM:SS of the measurement

my current select statement is below, I will mostly likely have to do separate statements for each variable but if I can avoid that I would like to.

SELECT StreamID,MeasurementID,MeasurementValue,MeasurementTime

  FROM measurements

  WHERE StreamID IN (26,27,28,29,30,31,32,33,34,35,14,36,37,38,39,40,
   41,42,43,44,45,46,47,48,49,50,18,13,11,12)

  ORDER BY MeasurementTime DESC, StreamID;

Here I have my reading of the db using result set, I'm not sure how to do multiple queries in a row to ignore "&&result2.next()"

 while(result.next())//&&result2.next()

 System.out.println(result.getString(1)+"\t"+result.getString(2)
 +"\t"+result.getString(3)+"\t"+result.getString(4)
 +"/t"+result.getString(5)+"\n";


con.close();

Answer 1

I'm not sure I understand exactly what you need to get from the db, but to execute a query hourly you can use a ScheduledExecutorService.

    ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
    Runnable task = () -> {
        // Run query
    };
    executor.scheduleAtFixedRate(task, 0, 1, TimeUnit.HOURS);

This will create a new thread that runs the run() method at a fixed rate. The rate is specified by scheduleAtFixedRate(task to run, initial delay (before the first time you run it), delay (between every time you run it), unit).

For the query you can try using a PreparedStatement so that you can just update the parameters instead of making a new query, thought I'm not completely sure what you want to do on that part, how do you know what IDs to look for?

EDIT: As I understand it, you're trying to get records hourly instead of every minute? The new Date and Time API has some pretty nifty tricks that can help you.

Here's a quick mockup I ran earlier:

String sql = "SELECT * FROM measurements ORDER BY Time";
PreparedStatement ps = conn.prepareStatement(sql);
ResultSet result = ps.executeQuery();
ArrayList<Integer> values = new ArrayList<>(); // This is where I store the values I want to keep, you can do it w/e way you want
LocalDateTime ldt1;
LocalDateTime ldt2;
// Take the earliest time
result.next(); // You might want to put that in an if() or catch Exceptions
ldt1 = LocalDateTime.parse(result.getString("Date"));
values.add(result.getInt("Value"));
// Increment throught the values until you find one with more than an hour difference with the first one, keep it and change that first one to this new one to continue
while (result.next()) {
    ldt2 = LocalDateTime.parse(result.getString("Date"));
    if (Duration.between(ldt1, ldt2).getSeconds() >= 60*60) {
        values.add(result.getInt("Value"));
        ldt1 = ldt2;
    }
}
System.out.println("values:\n");
System.out.println(values);

My simplified db has theses values:

Date                    | Value
2017-11-15T22:32:13.590 | 0
2017-11-15T22:47:13.590 | 1
2017-11-15T23:02:13.590 | 2
2017-11-15T23:17:13.590 | 3
2017-11-15T23:32:13.590 | 4
2017-11-15T23:47:13.590 | 5
2017-11-16T00:02:13.590 | 6
2017-11-16T00:17:13.590 | 7
2017-11-16T00:32:13.590 | 8
2017-11-16T00:47:13.590 | 9
2017-11-16T01:02:13.590 | 10
2017-11-16T01:17:13.590 | 11
2017-11-16T01:32:13.590 | 12
2017-11-16T01:47:13.590 | 13
2017-11-16T02:02:13.590 | 14
2017-11-16T02:17:13.590 | 15
2017-11-16T02:32:13.590 | 16
2017-11-16T02:47:13.590 | 17
2017-11-16T03:02:13.590 | 18
2017-11-16T03:17:13.590 | 19
2017-11-16T03:32:13.590 | 20
2017-11-16T03:47:13.590 | 21
2017-11-16T04:02:13.590 | 22
2017-11-16T04:17:13.590 | 23
2017-11-16T04:32:13.590 | 24
2017-11-16T04:47:13.590 | 25
2017-11-16T05:02:13.590 | 26
2017-11-16T05:17:13.590 | 27
2017-11-16T05:32:13.590 | 28
2017-11-16T05:47:13.590 | 29

And the values ArrayList contains:

[0, 4, 8, 12, 16, 20, 24, 28]

If you don't have Strings or whatnot you can also make conversions from TimeStamp to LocalDateTime.