Indexing very long number column - oracle

Question

I have a table with few columns including 2 varchar2(200) columns. In this columns we basically store serial numbers which can be numeric or alpha-numeric. Alpha-numeric values always both serials are same in those 2 columns. However for number serials it is a range like (first column value = 511368000004001226 and second column value = 511368000004001425 with 200 different (Qty)). Maximum length of the serial is 20 digits. I have indexed both the columns.
Now I want to sear a serial in-between the above range. (lets say 511368000004001227). I use following query.
SELECT *
FROM Table_Namr d
WHERE d.FROM_SN <= '511368000004001227'
AND d.TO_SN >= '511368000004001227'
Is it a valid query? Can I use <=> operators for numbers in a varchar column?

Answer 1

Yes, You can use >= and <= operators on Varchar2 columns but it will behave like it is string and comparison between strings will take place.
In this case, 4 will be considered greater than 34 means '4' > '34' but number 4 is less than 34.
It is not a good practice to store a number in Varchar2. You will lose the functionality of Numbers if you store them in varchar2.
You can check the above concept using following:
select * from dual where '4' > '34'; -- gives result 'X'
select * from dual where 4 > 34; -- Gives no result
You can try to convert the varchar2 column to number using to_number if possible in your case.
Cheers!!

Answer 2

Your Query is "valid" in the sense, that it works, and will deliver a result. If you are looking from a numeric standpoint, it will not work correctly, as the range operators for VARCHAR columns work the same way, as it would sort an alphanumeric value.
e.g.
d.FROM_SN >= '51000'
AND d.TO_SN <= '52000'
This would match for values, as you would expect, like 51001, 51700, but would also deliver unexpected values like 52, or 5100000000000000
If you want numeric selection, you would need to parse it - which of course only works, if every value in these columns is numeric:
TO_NUMBER(d.FROM_SN) >= 51000
AND TO_NUMBER(d.TO_SN) <= 52000

Answer 3

You may use alphanumerical comparison provided
1) your ranges are of the same length and
2) all the keys in the range are of the same length
Example data
SERNO
----------------------------------------
101
1011
1012
1013
1014
102
103
104
This doesn't work
select * from tab
where serno >= '101' and serno <= '102';
SERNO
----------------------------------------
101
102
1011
1012
1013
1014
But constraining the lentgh of the result provides the right answer
select * from tab
where serno >= '101' and serno <= '102'
and length(serno) = 3;
SERNO
----------------------------------------
101
102