数据库中查询，两个经纬度之间的距离代码

double radians(double degrees)
{
    return degrees * M_PI / 180.0;
}


- (void)testDistance
{
    FMDatabase *db = [FMDatabase databaseWithPath:_writableDBPath];
    if (![db open]) {
        return;
    }

    [db makeFunctionNamed:@"distance" maximumArguments:4 withBlock:^(sqlite3_context *context, int argc, sqlite3_value **argv) {
        double values[4];
        
        // get the double values for the four arguments
        
        for (int i = 0; i < 4; i++) {
            int dataType = sqlite3_value_numeric_type(argv[i]);
            
            if (dataType == SQLITE_INTEGER || dataType == SQLITE_FLOAT) {
                values[i] = sqlite3_value_double(argv[i]);
            } else {
                sqlite3_result_null(context);
                return;
            }
        }
        
        // let‘s give those values meaningful variable names
        
        double lat  = radians(values[0]);
        double lng  = radians(values[1]);
        double lat2 = radians(values[2]);
        double lng2 = radians(values[3]);
        
        // calculate the distance
        
        double result = 6371.393 * acos(cos(lat2) * cos(lat) * cos(lng2 - lng) + sin(lat2) * sin(lat));
        
        sqlite3_result_double(context, result);
    }];
    
    int rowCount = 0;
    FMResultSet *rs = [db executeQuery:@"SELECT *,distance(lat,lng,34.27344,108.95996) AS distance FROM hotel WHERE cityId = 10 AND lat > 33.27344 AND lat < 35.27344 AND lng > 107.95996 AND lng < 109.95996 ORDER BY distance ASC LIMIT 10"];
    
    while ([rs next]) {
        rowCount++;
        NSLog(@"Does %@,%@ ", [rs stringForColumnIndex:1],[rs stringForColumn:@"distance"]);
    }
}