Writing user defined SQL functions for SQLite using Java or Groovy?

Question

With SQLite, user defined SQL functions can easily be added using the C api or PHP. But is it also possible using Java or Groovy?

Answer 1

Simple way to create a function which accepts arguments and return result:

        Function.create(conn, "addNS", new Function() {
            @Override
            protected void xFunc() {
                System.out.println("myFunc called!");
                String arg1;
                try {
                    arg1 = value_text(0);
                    System.out.println("function arg1:"+arg1);
                    result("NS-"+arg1);
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        },  1, Function.FLAG_DETERMINISTIC);

        rs = conn.createStatement().executeQuery("select addNS('xyz');");
        while(rs.next()) {
            String val = rs.getString(1);
            System.out.println("Function return val : " + val);
        }

Answer 2

Since the question is about solution in Java or Groovy, someone can look here for Java solution (just like i did)

Below you can see simple function that validates, if provided dates are in same day:

public class IsSameDay extends Function {

    @Override
    protected void xFunc() throws SQLException {
        if (args() != 2) {
            throw new SQLException("IsSameDay(date1,date2): Invalid argument count. Requires 2, but found " + args());
        }
        try {
            DateTime t1 = DateTime.parse(value_text(0).replace(" ", "T"));
            DateTime t2 = DateTime.parse(value_text(1).replace(" ", "T"));
            if (t1.getYear() == t2.getYear() && t1.getDayOfYear() == t2.getDayOfYear()) {
                result(1);
            } else {
                result(0);
            }
        } catch (Exception exception) {
            throw new SQLDataException("IsSameDay(date1,date2): One of Arguments is invalid: " + exception.getLocalizedMessage());
        }
    }
}

and if someone needs to do an aggregation function, then might find this example useful:

public class MyMax extends Function.Aggregate {

    private long buff = 0;

    public MyMax() {
    }

    @Override
    protected void xStep() throws SQLException {
        long current = value_long(0);
        if (current > buff) {
            buff = current;
        }

    }

    @Override
    protected void xFinal() throws SQLException {
        result(buff);
    }
}

accumulator works because in each query MyMax instantion is cloned, so start value can be provided when instantiating object.

at the end example showing how to attach functions:

Function.create(c, IsSameDay.class.getSimpleName(), new IsSameDay());
Function.create(c, MyMax.class.getSimpleName(), new MyMax());

Hope that someone will find it useful.

Answer 3

It turns out writing a user defined function is actually quite easy using SQLiteJDBC. Here's a Groovy example:

@GrabConfig(systemClassLoader=true)
@Grab('org.xerial:sqlite-jdbc:3.6.16')
import org.sqlite.*
import java.sql.*

db = groovy.sql.Sql.newInstance("jdbc:sqlite::memory:","org.sqlite.JDBC")

// a distance function using the spherical law of cosines
Function.create(db.getConnection(), "distance", new Function() {
    protected void xFunc() throws SQLException {
        def lat1 = value_double(0)
        def lon1 = value_double(1)
        def lat2 = value_double(2)
        def lon2 = value_double(3)

        double theta = lon1 - lon2;
        double dist = (Math.sin(deg2rad(lat1)) * Math.sin(deg2rad(lat2))) + 
            (Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) * Math.cos(deg2rad(theta)))
        dist = Math.acos(dist)
        dist = rad2deg(dist)
        dist = dist * 60 * 1.1515
        dist = dist * 1.609344
        result(dist);
    }

    def deg2rad(deg) {
      deg * Math.PI / 180.0
    }

    def rad2deg(rad) {
      rad * 180.0 / Math.PI
    }
})

db.execute("CREATE TABLE city(name, lat, lon)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('New York City', 40.7143, -74.0060)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('San Francisco', 37.7749, -122.4194)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('Paris', 48.8567, 2.3510)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('Cologne', 50.9407, 6.9599)")

db.eachRow("SELECT a.name as a, b.name as b, distance(a.lat, a.lon, b.lat, b.lon) as d FROM city a, city b WHERE a.name != b.name ORDER BY d;") {
    println "Distance from ${it.a} to ${it.b}: ${it.d}km"
}

Answer 4

I think, this is a simple way to create custom function,

Class.forName("org.sqlite.JDBC");
Connection conn = DriverManager.getConnection("jdbc:sqlite:");
Function.create(conn, "myFunc", new Function() {
           protected void xFunc() {
               System.out.println("myFunc called!");
           }
       });        
conn.createStatement().execute("select myFunc();");