(Not (`X`) Is Null)

CREATE TABLE IF NOT EXISTS cities 
(
    id VARCHAR(4) NOT NULL PRIMARY KEY,
    city VARCHAR(20) NOT NULL,
    x_lat DECIMAL(5,2),
    y_lon DECIMAL(5,2),
    INDEX( city(6) )

) ENGINE=INNODB;

INSERT INTO cities (id, city, x_lat, y_lon)
VALUES 
('PL01', 'City01', 10, 10),
('PL02', 'City02', 10.25, 11), 
('PL03', 'City03', 10.25, 10.1), 
('PL04', 'City04', 10.85, 10), 
('PL05', 'City05', 10.25, 11.2), 
('PL06', 'City06', 11, 9), 
('PL07', 'City07', 11.25, 9.25);

// PL03 as test city
SELECT t0.id as id0, t0.city as city0, t0.x_lat as x0 , t0.y_lon as y0,
       t1.id as id1, t1.city as city1, t1.x_lat as x1, t1.y_lon as y1, 
       (t0.x_lat - t1.x_lat)*(t0.x_lat - t1.x_lat) + (t0.y_lon -t1.y_lon)*(t0.y_lon -t1.y_lon) as dist
        
       FROM (SELECT * FROM cities WHERE id='PL03') t0, cities t1
       
       WHERE (t1.x_lat IS NOT NULL) AND (t1.y_lon IS NOT NULL) AND 
             (  (t0.x_lat - t1.x_lat)*(t0.x_lat - t1.x_lat) + (t0.y_lon -t1.y_lon)*(t0.y_lon -t1.y_lon) <= 1  )
       
       ORDER BY dist;