How to retrieve latest instance using IF else condition in sqldeveloper - sql

Question

I have one student table which is given below
ROLL | STUDENT | SUBJECT | VERSION_ID |
1 A M 1
2 B M 2
3 C M 3
4 B S 1
5 D S 2
6 E E 1
7 F G 1
If there is atleast one record for SUBJECT='M' then retrieve latest record for SUBJECT 'M' based on version id.
ELSIF If there is atleast one record for SUBJECT='S' then retrieve latest record for SUBJECT 'S' based on version id.
ELSIF If there is atleast one record for SUBJECT='E' then retrieve latest record for SUBJECT 'E' based on version id.
Else DO NOT fetch any record.
Based on above condition I am trying to write a query or view which can satisfy all above condition and and give below output. Please suggest me possible solutions to get this.
Output
ROLL | STUDENT | SUBJECT | VERSION_ID |
3 C M 3
I have tried to get desired output using below query but I encountered with error. Please help me to solve this query with better performance.
SELECT * from ( IF EXISTS(select subject from STUDENT where SUBJECT='M') then
select * from STUDENT S1
inner join
(select S2.ROLL from STUDENT S2
where
S2.ROLL=(select S3.ROLL from STUDENT S3
where S3.SUBJECT='M'
and S3.VERSION_ID=(select MAX(S4.VERSION_ID) from STUDENT S4
GROUP BY S4.SUBJECT
HAVING S4.SUBJECT='M'))) S5
ON
S1.ROLL=S5.ROLL
ELSIF EXISTS(select subject from STUDENT where SUBJECT='S') then
select * from STUDENT S1
inner join
(select S2.ROLL from STUDENT S2
where
S2.ROLL=(select S3.ROLL from STUDENT S3
where S3.SUBJECT='S'
and S3.VERSION_ID=(select MAX(S4.VERSION_ID) from STUDENT S4
GROUP BY S4.SUBJECT
HAVING S4.SUBJECT='S'))) S5
ON
S1.ROLL=S5.ROLL
ELSIF EXISTS(select subject from STUDENT where SUBJECT='E') then
select * from STUDENT S1
inner join
(select S2.ROLL from STUDENT S2
where
S2.ROLL=(select S3.ROLL from STUDENT S3
where S3.SUBJECT='E'
and S3.VERSION_ID=(select MAX(S4.VERSION_ID) from STUDENT S4
GROUP BY S4.SUBJECT
HAVING S4.SUBJECT='E'))) S5
ON
S1.ROLL=S5.ROLL
ELSE
SELECT * FROM STUDENT WHERE 1=2);
TABLE CREATION:
Create table student (ROLL number,STUDENT varchar2(20),SUBJECT varchar2(20),VERSION_ID number );
Insert Data:
insert into STUDENT values(1,'A','M',1);
insert into STUDENT values(2,'B','M',2);
insert into STUDENT values(3,'C','M',3);
insert into STUDENT values(4,'B','S',1);
insert into STUDENT values(5,'D','S',2);
insert into STUDENT values(6,'E','E',1);
insert into STUDENT values(7,'F','G',1);
Thank You..

Answer 1

We can apply ROW_NUMBER to entire table, using the ordering logic you gave in your question. Subjects M, S, and E have priority, in that order, with the VERSION_ID breaking the tie. Rows with a subject not matching these three are assigned the lowest priority. Should there be no records with matching subjects, we filter out all records in the WHERE clause.
WITH cte AS (
SELECT ROLL, STUDENT, SUBJECT, VERSION_ID,
ROW_NUMBER() OVER (ORDER BY CASE WHEN SUBJECT = 'M' THEN 1
WHEN SUBJECT = 'S' THEN 2
WHEN SUBJECT = 'E' THEN 3
ELSE 4 END, VERSION_ID DESC END) rn
)
SELECT ROLL, STUDENT, SUBJECT, VERSION_ID
FROM cte
WHERE rn = 1 AND SUBJECT IN ('M', 'S', 'E')

Answer 2

You want right answer right? Try this :
SELECT TOP 1 #student.*
FROM student
JOIN (
SELECT MAX(VERSION_ID) VERSION_ID,SUBJECT
FROM student
WHERE SUBJECT = 'M'
GROUP BY SUBJECT
UNION ALL
SELECT MAX(VERSION_ID),SUBJECT
FROM student
WHERE SUBJECT = 'S'
GROUP BY SUBJECT
UNION ALL
SELECT MAX(VERSION_ID),SUBJECT
FROM student
WHERE SUBJECT = 'E'
GROUP BY SUBJECT
UNION ALL
SELECT MAX(VERSION_ID),SUBJECT
FROM student
WHERE SUBJECT = 'G'
GROUP BY SUBJECT
UNION ALL
SELECT NULL,NULL
) AS Dtls
ON (student.SUBJECT = Dtls.SUBJECT
AND student.VERSION_ID = Dtls.VERSION_ID)