Is it possible to do this task more efficiently? MySQL - mysql

Question

Currently, I have a task: for the presented table, to change students' order by replacing odd and even sequence (example below). There is only one condition, if the number of students is odd, that last number should not be changed. I currently wrote a code like this, however, for me, it seems pretty clumsy. How different and more efficient should this code be written?
CREATE TABLE student (id int, name varchar(128));
INSERT INTO student (id,name) VALUES
(1,'Aurimas'),
(2,'Darius'),
(3,'Eligijus'),
(4,'Giedrius'),
(5,'Justinas');
SELECT CASE
WHEN mod((SELECT id FROM student ORDER BY id DESC LIMIT 1),2) = 0 THEN
CASE
WHEN mod(id, 2) = 0 THEN id-1
ELSE id=id+1
END
ELSE
CASE
WHEN mod(id, 2) = 0 AND id <> (SELECT id FROM student ORDER BY id DESC LIMIT 1) THEN id-1
WHEN mod(id, 2) = 1 AND id <> (SELECT id FROM student ORDER BY id DESC LIMIT 1) THEN id+1
ELSE id
END
END AS new_id, name
FROM student
ORDER BY new_id ASC;
I have this:
id name
1 Aurimas
2 Darius
3 Eligijus
4 Giedrius
5 Justinas
And it should look like this:
id name
1 Darius
2 Aurimas
3 Giedrius
4 Eligijus
5 Justinas

Answer 1

You could group your ids by the value of (id+1) DIV 2. Then all you have to do is swap the minimum id with the maximum id in each pair of rows and make sure you handle the case when there are odd number of rows properly.
Something like this:
set #maxId = (select max(id) from student);
select
if (
#maxId = id and mod(id, 2) != 0,
id,
if (
mod((id+1), 2) = 0,
id+1,
id-1
)
) as id,
name
from student
order by id;
db<>fiddle

Answer 2

I'm not sure how you're defining 'efficient', but how about...
select ROW_NUMBER() OVER (
ORDER BY ceiling(id/2), id desc
) new_id,name from student
The ROW_NUMBER function has been supported in MySQL since version 8.0.
EDIT: I suppose something like will satisfy the quibblers...
WITH cte (id,n) AS (SELECT id, ROW_NUMBER() OVER (ORDER BY id) n FROM student)
SELECT x.name
, ROW_NUMBER() OVER (ORDER BY CEILING(cte.n/2), x.id desc) new_id
FROM student x
JOIN cte
ON cte.id = x.id;

Answer 3

I would recommend to use the below query because
it has many advantages here I am calculating only one max id select but in your case for every case, one select will execute this will reduce that time and only one max id will be used for the odd case
your odd and even operation is the same but in odd you are skipping last record so I added that condition
SET #MAXID = (SELECT MAX(ID) FROM STUDENT);
SELECT
CASE
WHEN MOD(#MAXID,2) != 0 AND ID=#MAXID THEN ID
ELSE CASE WHEN MOD(ID, 2) = 0 THEN ID-1 ELSE ID+1 END
END AS NEW_ID, NAME
FROM STACK_USER.STUDENT
ORDER BY NEW_ID ASC;
I think this would be useful in your case I have checked every condition in my local it will work properly
Screen Shot For Better Understanding: