I want to be able to perform an SQL SELECT (sqlite) on a table and ORDER BY each rows distance from an arbitrary point

Question

I have a sqlite database with a table full of geographic locations, each stored as a latitude and longitude value in degrees. I wanted to be able to perform a SQL SELECT on this table and ORDER BY each row’s distance from an arbitrary point. I’ve achieved this by using the defined custom sqlite function (distanceFunc) below.

Here’s the function, together with a convenience macro to convert from degrees to radians. This function is based on an online distance calculator, which makes use of the spherical law of cosines.

http://en.wikipedia.org/wiki/Spherical_law_of_cosines

"In spherical trigonometry, the law of cosines (also called the cosine rule for sides) is a theorem relating the sides and angles of spherical triangles, analogous to the ordinary law of cosines from plane trigonometry."

#define DEG2RAD(degrees) (degrees * 0.01745327) // degrees * pi over 180

The "distanceFunc" in (Location.m)

static void distanceFunc(sqlite3_context *context, int argc, sqlite3_value **argv) {
 // check that we have four arguments (lat1, lon1, lat2, lon2)
 assert(argc == 4);

 // check that all four arguments are non-null
 if (sqlite3_value_type(argv[0]) == SQLITE_NULL || sqlite3_value_type(argv[1]) == SQLITE_NULL || sqlite3_value_type(argv[2]) == SQLITE_NULL || sqlite3_value_type(argv[3]) == SQLITE_NULL) {
  sqlite3_result_null(context);
  return;
 }

 // get the four argument values
 double lat1 = sqlite3_value_double(argv[0]);
 double lon1 = sqlite3_value_double(argv[1]);
 double lat2 = sqlite3_value_double(argv[2]);
 double lon2 = sqlite3_value_double(argv[3]);

 // convert lat1 and lat2 into radians now, to avoid doing it twice below
 double lat1rad = DEG2RAD(lat1);
 double lat2rad = DEG2RAD(lat2);

 // apply the spherical law of cosines to our latitudes and longitudes, and set the result appropriately
 // 6378.1 is the approximate radius of the earth in kilometres
 sqlite3_result_double(context, acos(sin(lat1rad) * sin(lat2rad) + cos(lat1rad) * cos(lat2rad) * cos(DEG2RAD(lon2) - DEG2RAD(lon1))) * 6378.1);
}

I call my method "getDistanceBetweenLongLat" which is also in "Location.m" like this:

Used in my (AppDelegate.m):

Location *location = [[Location alloc] init];
location.latitude = -37.1134; //double property
location.longitude = 145.4254; //double property
[location getDistanceBetweenLongLat:[self dbPath]];

My "getDistanceBetweenLongLat" method in (Location.m):

- (void) getDistanceBetweenLongLat:(NSString *)dbPath {

    AppDelegate *_ad = (AppDelegate *)[[UIApplication sharedApplication] delegate];

    if (sqlite3_open([dbPath UTF8String], &database) == SQLITE_OK) {

        sqlite3_create_function(database, "distance", 4, SQLITE_UTF8, NULL, &distanceFunc, NULL, NULL);

        const char *sql = "select * from locations order by distance(latitude, longitude, ?, ?)";
        sqlite3_stmt *selectstmt;

        sqlite3_bind_double(selectStmt, 1, latitude);
        sqlite3_bind_double(selectStmt, 2, longitude);

        if(sqlite3_prepare_v2(database, sql, -1, &selectstmt, NULL) == SQLITE_OK) {

            while(sqlite3_step(selectstmt) == SQLITE_ROW) {

                NSInteger primaryKey = sqlite3_column_int(selectstmt, 0);
                Location *location = [[Location alloc] initWithPrimaryKey:primaryKey];
                location.latitude = sqlite3_column_double(selectstmt, 1);
                location.longitude = sqlite3_column_double(selectstmt, 2);
                location.title = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 3)];
                location.street1 = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 4)];
                location.street2 = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 5)];
                location.suburb = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 6)];
                NSInteger postCodeItem = sqlite3_column_int(selectstmt, 7);
                location.postcode = postCodeItem;

                location.isDirty = NO;

                [_ad.locations addObject:location];
                [location release];

            }
        }
    } else {
        //Even though the open call failed, close the database connection to release all the memory.
        sqlite3_close(database);
    }
}

This is calling the "distanceFunc" just fine. However when it gets to the statement where it checks that all four arguments are non-null they are all coming back as null.

However, when I change my sql statement above to the following:

const char *sql = "select * from locations order by distance(latitude, longitude, '?', '?')"; //single quotes added

The four arguments don't come back as null. However, the last two argument values are 0 when they should be the following, right?

location.latitude = -37.1134; //double property
location.longitude = 145.4254; //double property:

From "distanceFunc" in (Location.m):

double lat1 = sqlite3_value_double(argv[0]); // -17.7699 from 1st result in Locations table
double lon1 = sqlite3_value_double(argv[1]); // 245.1103 from 1st result in Locations table
double lat2 = sqlite3_value_double(argv[2]); // 0
double lon2 = sqlite3_value_double(argv[3]); // 0

I use pretty much the same method to get initial data to display and that particular array populates just fine. This time I just want my locations array (in _ad.locations) to contain the results ordered by distance instead so I can display them in a UITableView.

NOTE: The "distanceFunc" used can also be found at the following link: http://www.thismuchiknow.co.uk/?p=71&cpage=1#comment-30834

Answer 1

I use a variation, where latVal and longVal are my current location.:

NSString *sqlString = @"SELECT * FROM stores WHERE status = 1 AND distance(latitude, longitude, %f, %f, id) < 800";
sqlString = [NSString stringWithFormat:sqlString, latVal, longVal];

i also use a variable to read the distance value within the distanceFunc, changing the last two lines to the following:

float val = acos(sin(lat1rad) * sin(lat2rad) + cos(lat1rad) * cos(lat2rad) * cos(DEG2RAD(lon2) - DEG2RAD(lon1))) * 6378.1;
distanceVal = (float)val;

sqlite3_result_double(context, val);

I then save the distanceVal into my objects that fit the match, applying a sort to the items returned:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"distance" ascending:YES];
[ locations sortUsingDescriptors: [ NSArray arrayWithObject: sortDescriptor ] ];
[sortDescriptor release];