option = Console.ReadLine();

i think there are extra inverted commas at the end of the update statement

subquery is not the only way.
u can simply create another query and fetch the data from the other table.
alternatively u can also use data tables...

what r u trying to do..?
do u want the user to enter only numeric values?

first u have to think of a way to determine whether data is saved or not.
thn this factor can be used as a condition to determine if u want to run the above code or not.

Private Sub...

r u using windows authentication login

on button click event
me.close()
frmnewform.show()

this is the property of the datagridview

change the property
allow users to add row to false

wht is the error tht u r geting

lstData.items.add(datatable.rows("column name")(row).tostring)

either delete the complete row and add a new row for the qty to be added or else use insert statement and specify only the qty column

ok remove the try catch statement
and get the exact error tht u r getting

why have u commented
dc.close.
the error might be because the connection is already open and u r again trying to open it

u can use data table along with command builder to add data.
but u will have to create a new row in the data table.
the inserting part will be taken care by the command builder

wht is the error tht u r getting?

i also recommend using mdi form

if its the selected value tht u want to add
then try

cbocustomer.selecteditem.tostring

you can store each random no. in an array.
and when the next random no. is generated check whether it has been generated by checking the contents of the array.

regarding ur parameter question
"?" indicates that this is the parameter. it is just any value.
but the value is decided by some other control.
eg.

insert into customer values "abc"

this is...

u need to use queries to insert data.
eg.
u begin with establishing a connection with the statement

dim oledbcon As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data...