How to work even and odd number query

Question

Plz explain how below worked even and odd query
2 ) why used subquery after from and which time we can use(what time of out put)
even
select * FROM (SELECT ROW_NUMBER() OVER(ORDER BY stud_id) as row ,grno,stud_id from stud_Enrollment) d
 WHERE d.row %2=0   
odd
select * FROM (SELECT ROW_NUMBER() OVER(ORDER BY stud_id) as row ,grno,stud_id from stud_Enrollment) d
 WHERE d.row %2=1

Answer 1

Row_Number function returns the sequential number of a row
(By using partition of a result set, starting at 1 for the first row in each partition)
>> why used subquery after from and which time we can use(what time of out put)
When we need sequntial numbers, we can use row_number function. Put it into derived table and use this derived table after FROM clause (Same way it is used in your query)
CREATE TABLE stud_Enrollment (
grno int,
stud_id int,
stud_name varchar(20)
INSERT INTO stud_Enrollment
VALUES (101, 511, 'Dheeraj'), (112, 521, 'Vaibhav'), (132, 522, 'Lalit'), (124, 564, 'Amogh'), (143, 598, 'Sushrut')
SELECT * FROM stud_Enrollment
-- Result of your table
--grno stud_id stud_name
--101 511 Dheeraj
--112 521 Vaibhav
--132 522 Lalit
--124 564 Amogh
--143 598 Sushrut
-- Now we need to find out the rows which are at even position, ie row should be at position of 0,2,4,6,8 etc..
-- But we don't have sequential number here in your table result
-- So we can create one new column by using row_number function
SELECT
ROW_NUMBER() OVER (ORDER BY stud_id) AS row, --> additiona "row" column
grno,
stud_id,
stud_name
FROM stud_Enrollment
-- We got "row" column in below output
--row grno stud_id stud_name
--1 101 511 Dheeraj
--2 112 521 Vaibhav
--3 132 522 Lalit
--4 124 564 Amogh
--5 143 598 Sushrut
-- Now above table is used after FROM clause. It uses row column in WHERE part
SELECT
FROM (SELECT
ROW_NUMBER() OVER (ORDER BY stud_id) AS row,
grno,
stud_id,
stud_name
FROM stud_Enrollment) d
WHERE d.row % 2 = 0
--row grno stud_id stud_name
--2 112 521 Vaibhav
--4 124 564 Amogh
-Vaibhav Chaudhari