create table bom_master (ID int NOT NULL AUTO_INCREMENT,label int, product varchar(20),PRIMARY KEY (ID)); 

insert into bom_master (label,product)values(1,'Budwieser');
insert into bom_master (label,product)values(2,'Heineken');
insert into bom_master (label,product)values(2,'Miller');
insert into bom_master (label,product)values(3,'Castle lite');
insert into bom_master (label,product)values(4,'Castle lager');
insert into bom_master (label,product)values(4,'Haywards');
insert into bom_master (label,product)values(3,'Kingfisher');
insert into bom_master (label,product)values(4,'Feni');

As you can see the lable column which depicts the label of each product. The immediate next label is the child of the previous label.
just for here 2 is the child of 1, next 2 is the child of same 1, then 3 is the child of 2 (the one that is immediate predecessor of 3). then 4 is the child of 3, next 4 is the child of same 3, then 3 is the child of the 2 with Miller as the product. And this follows.

SCOTT@orcl_12.1.0.2.0> create table bom_master
  2    (ID	 int GENERATED ALWAYS AS IDENTITY NOT NULL,
  3  	label	 int,
  4  	product  varchar(20),
  5  	PRIMARY KEY (ID));

Table created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(1,'Budwieser');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(2,'Heineken');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(2,'Miller');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(3,'Castle lite');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(4,'Castle lager');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(4,'Haywards');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(3,'Kingfisher');

1 row created.

SCOTT@orcl_12.1.0.2.0> insert into bom_master (label,product)values(4,'Feni');

1 row created.

SCOTT@orcl_12.1.0.2.0> select * from bom_master order by id
  2  /

        ID      LABEL PRODUCT
---------- ---------- --------------------
         1          1 Budwieser
         2          2 Heineken
         3          2 Miller
         4          3 Castle lite
         5          4 Castle lager
         6          4 Haywards
         7          3 Kingfisher
         8          4 Feni

8 rows selected.

SCOTT@orcl_12.1.0.2.0> select label_parent, label_child, product_parent, product_child
  2  from   (select t1.label label_parent, t2.label label_child,
  3  		    t1.product product_parent, t2.product product_child,
  4  		    t2.id id_child,
  5  		    row_number () over (partition by t2.id order by t1.id desc) as rn
  6  	     from   bom_master t1 join bom_master t2
  7  	     on     (t1.label = t2.label - 1 and t1.id < t2.id)) t4
  8  where  rn = 1
  9  order  by id_child
 10  /

LABEL_PARENT LABEL_CHILD PRODUCT_PARENT       PRODUCT_CHILD
------------ ----------- -------------------- --------------------
           1           2 Budwieser            Heineken
           1           2 Budwieser            Miller
           2           3 Miller               Castle lite
           3           4 Castle lite          Castle lager
           3           4 Castle lite          Haywards
           2           3 Miller               Kingfisher
           3           4 Kingfisher           Feni

7 rows selected.

mysql> use mysql;
Database changed
mysql> create table bom_master (ID int NOT NULL AUTO_INCREMENT,label int, product varchar(20),PRIMARY KEY (ID));
Query OK, 0 rows affected (0.24 sec)

mysql> insert into bom_master (label,product)values(1,'Budwieser');
Query OK, 1 row affected (0.04 sec)

mysql> insert into bom_master (label,product)values(2,'Heineken');
Query OK, 1 row affected (0.03 sec)

mysql> insert into bom_master (label,product)values(2,'Miller');
Query OK, 1 row affected (0.05 sec)

mysql> insert into bom_master (label,product)values(3,'Castle lite');
Query OK, 1 row affected (0.06 sec)

mysql> insert into bom_master (label,product)values(4,'Castle lager');
Query OK, 1 row affected (0.06 sec)

mysql> insert into bom_master (label,product)values(4,'Haywards');
Query OK, 1 row affected (0.03 sec)

mysql> insert into bom_master (label,product)values(3,'Kingfisher');
Query OK, 1 row affected (0.03 sec)

mysql> insert into bom_master (label,product)values(4,'Feni');
Query OK, 1 row affected (0.03 sec)

mysql> select * from bom_master;
+----+-------+--------------+
| ID | label | product      |
+----+-------+--------------+
|  1 |     1 | Budwieser    |
|  2 |     2 | Heineken     |
|  3 |     2 | Miller       |
|  4 |     3 | Castle lite  |
|  5 |     4 | Castle lager |
|  6 |     4 | Haywards     |
|  7 |     3 | Kingfisher   |
|  8 |     4 | Feni         |
+----+-------+--------------+
8 rows in set (0.00 sec)

mysql> select label_parent, label_child, product_parent, product_child
    -> from   (select t3.*,
    ->                @row_num := IF(@prev_value=t3.id_child,@row_num+1,1) as rn,
    ->                @prev_value := t3.id_child
    ->         from   (select t1.label label_parent, t2.label label_child,
    ->                        t1.product product_parent, t2.product product_child,
    ->                        t1.id id_parent, t2.id id_child
    ->                 from   bom_master t1 join bom_master t2
    ->                 on     (t1.label = t2.label - 1 and t1.id < t2.id)) t3,
    ->                (select @row_num := 1) x,
    ->                (select @prev_value := '') y
    ->         order  by t3.id_child, t3.id_parent desc) t4
    -> where  rn = 1
    -> order  by id_child;
+--------------+-------------+----------------+---------------+
| label_parent | label_child | product_parent | product_child |
+--------------+-------------+----------------+---------------+
|            1 |           2 | Budwieser      | Heineken      |
|            1 |           2 | Budwieser      | Miller        |
|            2 |           3 | Miller         | Castle lite   |
|            3 |           4 | Castle lite    | Castle lager  |
|            3 |           4 | Castle lite    | Haywards      |
|            2 |           3 | Miller         | Kingfisher    |
|            3 |           4 | Kingfisher     | Feni          |
+--------------+-------------+----------------+---------------+
7 rows in set (0.01 sec)