Mysql: update current value if previous row are same with current row

Question

How do i update current row value if my previous row are same with current row.

example:

the curent row is 68, previous row is also 68.. i would i like to update current row become 68-20 which is 48.

same for 98-20 = 78.

so that the corrected data will look like:

i have more than 1000 record like this, which cant update the record one by one manually.

update table1 set DIH_QTY_BALANCE=DIH_QTY_BALANCE-DIH_REORDER_QTY

WHERE how to put the previous row same as current on where clause?

Here is the Schema + data: http://pastebin.com/T1tYDT6Y

too large for sqlfiddle.

any help would be great.

Answer 1

As far as I remember, MySQL has problems to select from the same table in an update statement. And this is what you would have to do, because in order to update a record or not, you'd have to select its previous record from the same table.

So create a temporary table, give it row numbers, then select from it with a self join, to compare each record with its previous record.

create temporary table temp
(
  rownum int, 
  dihistoryid int, 
  dih_qty_balance int
) engine = memory;

set @num = 0;

insert into temp
  select 
    @num := @num + 1 as rownum, 
    dihistoryid, 
    dih_qty_balance 
  from mytable
  order by dihistoryid;

update mytable
set dih_qty_balance = dih_qty_balance - dih_reorder_qty
where dihistoryid in
(
  select current.dihistoryid
  from temp current
  join temp previous on previous.rownum = current.rownum - 1
  where previous.dih_qty_balance = current.dih_qty_balance
);

drop temporary table temp;

Answer 2

May be something like this

SELECT DIH_QTY_BALANCE,
       (SELECT DIH_QTY_BALANCE FROM example e2
        WHERE e2.DIHISTORYID < e1.DIHISTORYID
        ORDER BY DIHISTORYID DESC LIMIT 1) as previous_value,
       (SELECT value FROM example e3
        WHERE e3.DIHISTORYID > e1.DIHISTORYID 
        ORDER BY DIHISTORYID ASC LIMIT 1) as next_value
FROM example e1