Sequence generation for new ID - sql

Question

I have a requirement where there are 2 tables A and B. There is a column named ID which is a primary key for A and foreign key for table B where the tables have one to many relationship between them. In table A for ID column we have entries 1,2,3 and for them we have corresponding multiple entries for one ID and another column in table B named SEQ where a sequence has been created for entries in it. Now for entries for ID 1, I have 3 entries in table B with SEQ 1,2,3 but when new ID entry will be there then i need the sequence again to start from 1 for that ID.**Can you please help me to do that.
TABLE A

Answer 1

I'd suggest you not to store the SEQ value. Why would you? It is easy to calculate it whenever needed. How? Like this, using row_number analytic function:
SQL> with b (id, name) as
2 (select 1, 'TRI' from dual union all
3 select 1, 'TRI' from dual union all
4 select 1, 'TRI' from dual union all
5 select 2, 'ROHIT' from dual union all
6 select 2, 'ROHIT' from dual union all
7 select 3, 'RAVI' from dual
8 )
9 select id,
10 name,
11 row_number() over (partition by id order by null) seq
12 from b;
ID NAME SEQ
---------- ----- ----------
1 TRI 1
1 TRI 2
1 TRI 3
2 ROHIT 1
2 ROHIT 2
3 RAVI 1
6 rows selected.
SQL>
If you still want to store it, now you know how.

Answer 2

Don't have multiple sequences. Just use a single sequence in the B table and accept that there will be gaps for each ID and then if you need sequential values you can calculate them as needed using the ROW_NUMBER() analytic function and, if you need to, put it in a view. Also, don't duplicate the name from table A in table B; keep your data normalised.
CREATE TABLE A (
id NUMBER(8,0)
GENERATED ALWAYS AS IDENTITY
CONSTRAINT A__id__pk PRIMARY KEY,
name VARCHAR2(20)
);
CREATE TABLE B (
id NUMBER(8,0)
CONSTRAINT B__id__nn NOT NULL
CONSTRAINT B__id__fk REFERENCES A(id),
seq NUMBER(8,0)
GENERATED ALWAYS AS IDENTITY
CONSTRAINT B__seq__pk PRIMARY KEY
);
Then you can create your sample data:
INSERT INTO A ( name )
SELECT 'TRI' FROM DUAL UNION ALL
SELECT 'ROHIT' FROM DUAL UNION ALL
SELECT 'RAVI' FROM DUAL;
INSERT INTO B ( id )
SELECT 1 FROM DUAL UNION ALL
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 1 FROM DUAL;
And:
SELECT *
FROM B
Outputs:
ID | SEQ
-: | --:
1 | 1
1 | 2
2 | 3
3 | 4
2 | 5
1 | 6
If you want your output then create a view:
CREATE VIEW B_view ( id, name, seq ) AS
SELECT b.id,
a.name,
ROW_NUMBER() OVER ( PARTITION BY b.id ORDER BY seq )
FROM B
INNER JOIN A
ON ( B.id = A.id )
Then:
SELECT *
FROM b_view
Outputs:
ID | NAME | SEQ
-: | :---- | --:
1 | TRI | 1
1 | TRI | 2
1 | TRI | 3
2 | ROHIT | 1
2 | ROHIT | 2
3 | RAVI | 1
db<>fiddle here