How to update each data of a column? - sql

Question

I have a table named class
|Record_No [PK] | Student_No | Class_No | Seat_No |
=====================================================
|1 | 200910 | 2 | 20 |
|2 | 201234 | 2 | 13 |
|3 | 200965 | 2 | 1 |
|4 | 200920 | 2 | 8 |
|5 | 200911 | 2 | 9 |
|6 | 200955 | 1 | 10 |
|7 | 200924 | 1 | 9 |
|8 | 200922 | 1 | 1 |
|9 | 200901 | 2 | 11 |
|10 | 200902 | 2 | 18 |
is it possible to update the numbers in Seat_No in order from 1 up to the last count of Seat No where its class no is equal to 2?
the table should look like this:
|Record_No [PK] | Student_No | Class_No | Seat_No |
=====================================================
|1 | 200910 | 2 | 1 |
|2 | 201234 | 2 | 2 |
|3 | 200965 | 2 | 3 |
|4 | 200920 | 2 | 4 |
|5 | 200911 | 2 | 5 |
|6 | 200955 | 1 | 10 |
|7 | 200924 | 1 | 9 |
|8 | 200922 | 1 | 1 |
|9 | 200901 | 2 | 6 |
|10 | 200902 | 2 | 7 |
as of now, i can only achieve this by using
UPDATE class SET Seat_No = 1 WHERE Class_No = 2 AND Student_No = 200910;
UPDATE class SET Seat_No = 2 WHERE Class_No = 2 AND Student_No = 201234;
...
and so on..
How can i solve this without putting every student_no in the query? pls help.

Answer 1

update class
set seat_no = s.rn
from (
select
row_number() over(order by record_no) rn,
record_no
from class
where class_no = 2
) s
where class.record_no = s.record_no
If you want to update all classes:
update class
set seat_no = s.rn
from (
select
row_number() over(partition by class_no order by record_no) rn,
record_no
from class
) s
where class.record_no = s.record_no

Answer 2

Assuming your Record_No is 1-5 in your example, you can use it:
UPDATE class SET Seat_No = Record_No WHERE Class_No = 2
If not, it depends on your RDBMS.
Here is the postgresql approach:
UPDATE Class C
SET Seat_No = t.rn
FROM (
SELECT Record_No, ROW_NUMBER() OVER (ORDER BY Record_No) rn
FROM Class
WHERE Class_no = 2
) t
WHERE C.Record_No = t.Record_No
SQL Fiddle Demo