PL SQL Group By Issue - oracle

Question

I have two tables T1 and T2.
T1 has an ID column that is generated as a sequence.
It also has two columns first name and Last name.
The table T2 is connected to table T1 via the ID column (referential).
T2 table has a salary column, that is revised every few years.
I want to get all the first name, last name, salary , and salary date if the salary has changed.
I am not able to get this information using the ID.
A second ID is generated for the same FN and LN pair if the employee comes up for review.
For Example :-
ID FN LN
1 John Doe
2 John Doe
ID SALARY DATE
1 $1 2015
2 $2 2018
I am trying something like this
SELECT T.FN ||' '|| T.LN AS NAME, COUNT(*) AS CT,
S.SALARY, S.DATE
SALARYTABLE S, EMP T
WHERE S.ID=T.ID
HAVING COUNT(*) > 1
GROUP BY (T.FN ||' '|| T.LN);
I have solved this by using a Java program. I have to store all the ID's and loop through all the records and check if the FN and LN matches and then extract the Date and Salary. This is inefficient and I want to do it within PL/SQL.
Please help. Thank you.

Answer 1

Well, your data model is kind of wrong; you shouldn't rely on distinguishing people on their names. What if yet another "John Doe" gets employed?
Anyway: would something like this do?
CTEs T1 and T2 simulate your tables. I added some more rows, just to make sure that the following query doesn't fail too obviously
INTER CTE joins those two tables and calculates employee's previous salary (using the LAG function)
the final query select rows (from INTER) whose current and previous salary are different
As you already have those tables, you'd use lines 16 onwards.
SQL> with
2 t1 (id, fn, ln) as
3 (select 1, 'John', 'Doe' from dual union all
4 select 2, 'John', 'Doe' from dual union all
5 select 3, 'John', 'Doe' from dual union all
6 select 5, 'Billy', 'Jean' from dual union all
7 select 6, 'Billy', 'Jean' from dual
8 ),
9 t2 (id, salary, c_date) as
10 (select 1, 1, 2015 from dual union all
11 select 2, 2, 2018 from dual union all
12 select 3, 2, 2019 from dual union all
13 select 5, 3, 2016 from dual union all
14 select 6, 3, 2017 from dual
15 ),
16 inter as
17 (select
18 t1.id, t1.fn, t1.ln,
19 t2.id, t2.salary, t2.c_date,
20 lag(t2.salary) over (partition by t1.fn, t1.ln
21 order by c_date) prev_salary
22 from t1 join t2 on t1.id = t2.id
23 )
24 select i.fn, i.ln, i.salary, i.c_date
25 from inter i
26 where i.salary <> nvl(i.prev_salary, i.salary)
27 order by i.ln, i.c_date;
FN LN SALARY C_DATE
----- ---- ---------- ----------
John Doe 2 2018
SQL>