need to calculate percentage from table data

Bad idea.  If this is Excel, no problem.  If you are doing this in the table itself you are violating good normalization rules.

Standard example is: Age and Birthdate in the same table is bad because one is a direct calculation of the other.  You could end up with them getting out of sync.  It is just unnecessary.  Do the calculation in the application(s) that use the table.

k sure, but please give me a work out, creating a new temp table with this column and doing the calculation there?

what you can do is that have a stored procedure / a view to do the required calculations

View or stored procedure

The basic select for your % per customer.

select Cust_no,   100 * (sum(case when On-time= 'O' then 1 else 0 end) / count(Cust_no)) as Pct
from dbo.tableA
group by Cust_no

Select allOpen in new window

Views are excellent for this sort of thing.  The underlying data stays pristine.

thanks the reply but I am getting result as 0 or 100

How to debug .... (or how to start builiding it yourself)  start part by part implementing the functions you want and test step by step

Is the Case ok

select Cust_no,   On-time,
case when On-time= 'O' then 1 else 0 end,
from dbo.tableA

What count and sum + together in calculation

select Cust_no,  
count(Cust_no),
sum(case when On-time= 'O' then 1 else 0 end),
100 * (sum(case when On-time= 'O' then 1 else 0 end) / count(Cust_no)) as Pct
from dbo.tableA
group by Cust_no

your current query is doing integer division, need to convert numbers to decimal/float.
try this

select Cust_no,   100.0 * (sum(case when On-time= 'O' then 1 else 0 end) * 1.0 / count(Cust_no)) as Pct
from dbo.tableA
group by Cust_no

Select allOpen in new window

try

select Cust_no,   100.0 * (sum(case when On-time= 'O' then 1.0 else 0.0 end) / count(Cust_no)) as Pct
from dbo.tableA
group by Cust_no

thanks