Re: Compare rows

On 13 July 2011 07:15, LALIT KUMAR <lalit(dot)jss(at)gmail(dot)com> wrote:
> Hi,
> I have the following table.
> city    Rate         flag
> A         10             0
> A         20             1
> A         30              0
> A          2              0
> A         23              1
> A        12               0
> B          5               1
> B          43              0
> C         23                1
> C        67                 1
> For every city I have so set the  flag as -1 in row which is 0 above the row
> having flag as 1.
> So my out would be:
> city    Rate         flag
> A         10             -1
> A         20             1
> A         30              0
> A          2              -1
> A         23              1   (not changed to -1 as it not 0)
> A        12               0  (not changed to -1 because i need to compare
>  it to row of same city. so being last row not compared with any row)
> ----------------------------------------------------
> B          5               1
> B          43
> C         23                1
> C        67                 1

You could try this:

UPDATE
my_table
SET
flag = -1
FROM (
SELECT
city, dat, lead(flag, 1, 0) OVER (PARTITION BY city ORDER BY
dat) AS next_flag
FROM
my_table
) t2
WHERE
my_table.city = t2.city
AND
my_table.dat = t2.dat
AND
flag = 0
AND
next_flag = 1

This joins the target table with a subquery that uses a window
function to work out what the next value of the date column is, and
updates based on that.

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Thom Brown
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