Next row and previous row in the table - LAG or Lead function

LAG or Lead function 

select order_no, installment_id, installment_id2, due_date , due_date2,number_of_days

from 

(

select t.order_no, 

        installment_id,          

        LEAD (installment_id,1) OVER (PARTITION BY t.order_no ORDER BY installment_id) AS installment_id2,

        due_date,

        LEAD (due_date,1) OVER (PARTITION BY t.order_no ORDER BY due_date) AS due_date2,

        

        (LEAD (due_date,1) OVER (PARTITION BY t.order_no ORDER BY due_date)-due_date) number_of_days

  from C_CREDIT_PAYMENT_PLAN p , PAYMENT_PLAN c, CUSTOMER_ORDER_CFV t

where p.invoice_id=c.invoice_id

and p.order_no=t.order_no

and c_early_settle <>'TRUE'

and PARTY_TYPE_DB='CUSTOMER'

--and t.order_no='L500592'

and CUSTOMER_ORDER_API.GET_C_LEASING_ACCOUNT_STATUS(t.ORDER_NO)<>'Closed'

and installment_id>1

and CUSTOMER_ORDER_API.GET_ORDER_ID(t.ORDER_NO) <>'MIG'

) 

where number_of_days>31

//////

select order_no, installment_id, installment_id2, due_date , due_date2,number_of_days

from 

(

select t.order_no, 

        installment_id,          

        LAG (installment_id,1) OVER (PARTITION BY t.order_no ORDER BY installment_id) AS installment_id2,

        due_date,

        LAG (due_date,1) OVER (PARTITION BY t.order_no ORDER BY due_date) AS due_date2,

        

        (LAG (due_date,1) OVER (PARTITION BY t.order_no ORDER BY due_date)-due_date) number_of_days

  from C_CREDIT_PAYMENT_PLAN p , PAYMENT_PLAN c, CUSTOMER_ORDER_CFV t

where p.invoice_id=c.invoice_id

and p.order_no=t.order_no

and c_early_settle <>'TRUE'

and PARTY_TYPE_DB='CUSTOMER'

and t.order_no='L500592'

and CUSTOMER_ORDER_API.GET_C_LEASING_ACCOUNT_STATUS(t.ORDER_NO)<>'Closed'

and installment_id>1

and CUSTOMER_ORDER_API.GET_ORDER_ID(t.ORDER_NO) <>'MIG'

) 

--where number_of_days>31