Change Value based on previous rows in mysql

Question

I have a table which has below structure. I'm using phpmyadmin 3.4.5, mysql version 5.5.16.

Table

Invoice_ID - PO_ID- Product - Quantity  Invoice_Qty - Amount 
Inv1       - PO1  -  P1     -  100      - 50        - 1000      
Inv2       - PO1  -  P1     -  100      - 50        - 1000
Inv3       - PO2  -  P2     -  50       - 20        -  500
Inv4       - PO2  -  P2     -  50       - 20        -  500
Inv5       - PO2  -  P3     -  50       - 10        -  250

What I'm really want to do is that

If Previous Row of PO_ID and and Product Name is Same as current Row PO_ID and Product then Quantity of current row should be zero?

Sum of Invoice_Quantity = Quantity. So Required like below

My Expected Output given below:

Out Put:
Invoice_ID - PO_ID- Product - Quantity  Invoice_Qty - Amount 
Inv1       - PO1  -  P1     -  100      - 50        - 1000      
Inv2       - PO1  -  P1     -  0        - 50        - 1000
Inv3       - PO2  -  P2     -  50       - 20        -  500
Inv4       - PO2  -  P2     -  0        - 20        -  500
Inv5       - PO2  -  P3     -  0        - 10        -  250

I tried the How to get result set like Oracle lag function. But It not worked for me.

And tried to write a procedure for that. I'm stuck with export resutlset. That is I don't know how to assign and get the result set.

Please help me out this problem.

Refer: http://sqlfiddle.com/#!2/5c0b0/4

Answer 1

Your sqlfiddle was confusing. Please don't provide sample data here and then use different sample data in the sqlfiddle. And your desired result here is wrong, since you said in the description

If Previous Row of PO_ID and and Product Name is Same as current Row PO_ID and Product then Quantity of current row should be zero

Anyway, used my own...

select
t.*,
if(@previd = po_id and @prevprod = Product, 0, Quantity) AS new_quantity,
@previd := po_id,
@prevprod := product
from
t
, (select @previd:=null, @prevprod:=null) var_init
order by po_id, product

• sqlfiddle

Note, that the order in the select clause is important, as well as the order by clause.

Answer 2

The previous ID is the maximum ID of all lower IDs. So the statement can be written as:

select 
  invoice_id, po_id, product,
  case when mytable.po_id = prev_mytable.po_id and mytable.product = prev_mytable.product
    then 0 
    else mytable.quantity 
  end as qty,
  invoice_qty, amount 
from mytable
left join mytable prev_mytable on prev_mytable.id = 
(
  select max(id) 
  from mytable all_prev_mytable 
  where all_prev_mytable.id < mytable.id
)
order by invoice_id;

And here is the SQL fiddle: http://sqlfiddle.com/#!2/5c0b0/11.

This is Standard SQL and should thus work with about any dbms.

Answer 3

this works :):) :

select Invoice_ID,PO_ID,product,
case when 
decode(lead(Quantity) over (order by PO_ID),Quantity,'SAME','DIFF') = 'SAME'
then Quantity
else 0
end Quantity, Amount 
from <table-name>