Oracle, If-else condition in where clause [duplicate] - oracle

Question

I have a table, A and B which are shown below,
Table
A:
id
idB
name
faculty
B:
id
name
Table B has 2 records as below.
SELECT *
FROM B;
1, 1, 'First'
2, 2, 'Second'
Table A has 8 records as below.
SELECT *
FROM A;
1, 1, A, IT
2, 1, B, IT
3, 1, C, IT
4, 1, D, Medicine
5, 1, E, Medicine
6, 1, F, Business
7, 1, G, Business
8, 1, H, IT
9, 2, A, Medicine
10, 2, B, Medicine
11, 2, C, Medicine
12, 2, D, Medicine
13, 2, E, Medicine
14, 2, F, Medicine
15, 2, G, Business
16, 2, H, Medicine
My question is:
How can I select data from table B where faculty should be IT and if there are multiple it should get with max ID. AND if there is no any IT, it should be get business?
My select view should be look like this below:
A and B records.
8, 1, H, IT, First
15, 2, G, Business, Second
Please could you advise and help in which way we can retrieve these data?

Answer 1

this query will help you to get your desired result
SELECT id
,name
,faculty
FROM A
WHERE faculty IN ('IT', 'Business')

Answer 2

How can I select data from table B where faculty should be IT and if there are multiple it should get with max ID. AND if there is no any IT, it should be get business?
This will get the row with the maximum ID that is in IT and if there are no IT rows then Business
SELECT *
FROM (
SELECT A.id,
A.idB,
A.name,
A.faculty,
B.name AS bname
FROM A
INNER JOIN B
ON ( A.idB = B.id )
WHERE A.faculty IN ( 'IT', 'Business' )
ORDER BY
DECODE( A.faculty, 'IT', 1, 'Business', 2 ),
A.id DESC
)
WHERE ROWNUM = 1;
If you want the top row from each group then:
SELECT id,
idB,
name,
faculty,
bname
FROM (
SELECT A.id,
A.idB,
A.name,
A.faculty,
B.name AS bname,
ROW_NUMBER() OVER (
PARTITION BY A.faculty
ORDER BY A.id DESC
) AS rn
FROM A
INNER JOIN B
ON ( A.idB = B.id )
WHERE A.faculty IN ( 'IT', 'Business' )
)
WHERE rn = 1;

Answer 3

SELECT A.*,T.Name
FROM
TableA A
INNER JOIN
(
SELECT MAX(A.id) AS id,A.FACULTY,B.NAME,
ROW_NUMBER() OVER (PARTITION BY B.NAME ORDER BY MAX(A.id) DESC) AS RN FROM TableA A
INNER JOIN TableB B
ON A.idB=B.id
WHERE faculty IN ('IT', 'Business')
GROUP BY A.FACULTY,B.NAME
) T
ON A.id = T.id
AND A.FACULTY = T.FACULTY
WHERE T.RN=1
Output
ID IDB NAME FACULTY NAME
8 1 H IT First
15 2 G Business Second
Demo
http://sqlfiddle.com/#!4/98b84/24