Oracle 18c

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CREATE TABLE towers (
  tower_id PRIMARY KEY,
  t_lat,
  t_long
) AS
SELECT 'CM0321', 6.890487, 79.869199 FROM DUAL UNION ALL
SELECT 'BD0010', 6.947618, 81.160246 FROM DUAL UNION ALL
SELECT 'CM0037', 6.896111, 79.868611 FROM DUAL UNION ALL
SELECT 'GM0121', 7.121666, 80.028888 FROM DUAL;
4 rows affected

CREATE TABLE customers (
  customer_number PRIMARY KEY,
  cx_lat,
  cx_long,
  latched_tower_id
) AS
SELECT 34532, 6.897257333, 79.86474533, 'CM0321' FROM DUAL UNION ALL
SELECT 43445, 6.935598403, 81.14939421, 'BD0010' FROM DUAL UNION ALL
SELECT 54365, 6.866224000, 79.88215000, 'CM0037' FROM DUAL UNION ALL
SELECT 52568, 7.113198000, 80.03724700, 'GM0121' FROM DUAL;
4 rows affected

ALTER TABLE customers ADD CONSTRAINT customers__lti__fk
  FOREIGN KEY (latched_tower_id) REFERENCES towers (tower_id);
CREATE FUNCTION haversine_distance(
  lat1  IN NUMBER,
  long1 IN NUMBER,
  lat2  IN NUMBER,
  long2 IN NUMBER
) RETURN NUMBER DETERMINISTIC
IS
  PI           CONSTANT NUMBER := ASIN(1) * 2;
  R            CONSTANT NUMBER := 6371000; -- Approx. radius of the earth in m
  PHI1         CONSTANT NUMBER := lat1 * PI / 180;
  PHI2         CONSTANT NUMBER := lat2 * PI / 180;
  DELTA_PHI    CONSTANT NUMBER := (lat2 - lat1) * PI / 180;
  DELTA_LAMBDA CONSTANT NUMBER := (long2 - long1) * PI / 180;
  a NUMBER;
  c NUMBER;
BEGIN
  a := SIN(delta_phi/2) * SIN(delta_phi/2) + COS(phi1) * COS(phi2) *
          SIN(delta_lambda/2) * SIN(delta_lambda/2);
  c := 2 * ATAN2(SQRT(a), SQRT(1-a));
  RETURN R * c; -- in metres
END;
/
SELECT customer_number,
       cx_lat,
       cx_long,
       latched_tower_id,
       distance,
       CASE
       WHEN latched_tower_id != closer_tower_id
       THEN closer_tower_id
       END AS closer_tower_id,
       CASE
       WHEN latched_tower_id != closer_tower_id
       THEN closer_distance
       END AS closer_distance
FROM   (
  SELECT c.*,
         TO_CHAR(
           HAVERSINE_DISTANCE(c.cx_lat, c.cx_long, t.t_lat, t.t_long)/1000,
           'FM999990.000'
         ) AS distance,
         ct.tower_id AS closer_tower_id,
         TO_CHAR(
           HAVERSINE_DISTANCE(c.cx_lat, c.cx_long, ct.t_lat, ct.t_long)/1000,
           'FM999990.000'
         ) AS closer_distance,
         ROW_NUMBER() OVER (
           PARTITION BY c.customer_number
           ORDER BY HAVERSINE_DISTANCE(c.cx_lat, c.cx_long, ct.t_lat, ct.t_long)
         ) AS rn
  FROM   customers c
         INNER JOIN towers t
         ON (t.tower_id = c.latched_tower_id)
         CROSS JOIN towers ct
  ORDER BY
         customer_number,
         DISTANCE ASC
)
WHERE  rn = 1;


CUSTOMER_NUMBER
CX_LAT
CX_LONG
LATCHED_TOWER_ID
DISTANCE
CLOSER_TOWER_ID
CLOSER_DISTANCE


34532
6.897257333
79.86474533
CM0321
0.899
CM0037
0.445

43445
6.935598403
81.14939421
BD0010
1.795
null
null

52568
7.113198
80.037247
GM0121
1.318
null
null

54365
6.866224
79.88215
CM0037
3.644
CM0321
3.053


SELECT c.*,
       TO_CHAR(
         HAVERSINE_DISTANCE(c.cx_lat, c.cx_long, t.t_lat, t.t_long)/1000,
         'FM999990.000'
       ) AS distance,
       ct.tower_id AS closer_tower_id,
       TO_CHAR(ct.distance, 'FM999990.000') AS closer_distance
FROM   customers c
       INNER JOIN towers t
       ON (t.tower_id = c.latched_tower_id)
       LEFT OUTER JOIN LATERAL(
         SELECT ct.*,
                HAVERSINE_DISTANCE(
                    c.cx_lat,
                    c.cx_long,
                    ct.t_lat,
                    ct.t_long
                )/1000 AS distance
         FROM   towers ct
         ORDER BY distance ASC
         FETCH FIRST ROW ONLY
       ) ct
       ON (ct.tower_id != c.latched_tower_id);


CUSTOMER_NUMBER
CX_LAT
CX_LONG
LATCHED_TOWER_ID
DISTANCE
CLOSER_TOWER_ID
CLOSER_DISTANCE


43445
6.935598403
81.14939421
BD0010
1.795
null
null

54365
6.866224
79.88215
CM0037
3.644
CM0321
3.053

34532
6.897257333
79.86474533
CM0321
0.899
CM0037
0.445

52568
7.113198
80.037247
GM0121
1.318
null
null
CUSTOMER_NUMBER	CX_LAT	CX_LONG	LATCHED_TOWER_ID	DISTANCE	CLOSER_TOWER_ID	CLOSER_DISTANCE
34532	6.897257333	79.86474533	CM0321	0.899	CM0037	0.445
43445	6.935598403	81.14939421	BD0010	1.795	null	null
52568	7.113198	80.037247	GM0121	1.318	null	null
54365	6.866224	79.88215	CM0037	3.644	CM0321	3.053