Previous row end date as the next row start date in SQL - sql

Question

Need Some help Please,
I have a Field called 'hist_lastupdated' that contains the last updated date of the modification of the price of a product.
Based in this field, i want to extract the start date and the end date of the modification.
In fact i have this:
**Product_id , Price , hist_lastupdated**
284849 18.95 2015-05-29 00:53:55
284849 15.95 2015-08-14 01:04:46
284849 18.95 2016-06-11 00:50:31
284849 15.95 2016-08-24 00:45:11
And i want to get the result like that :
**Product_id , Price , hist_lastupdated ,start_date , End_date**
284849 18.95 2015-05-29 00:53:55 2014-05-01 00:00:00 2015-05-29 00:53:55
284849 15.95 2015-08-14 01:04:46 2015-05-29 00:53:55 2015-08-14 01:04:46
284849 18.95 2016-06-11 00:50:31 2015-08-14 01:04:46 2016-06-11 00:50:31
284849 15.95 2016-08-24 00:45:11 2016-06-11 00:50:31 2016-08-24 00:45:11
In two word, the start date is the end date of the previous line
i have many product id

Answer 1

Something like this:
select Product_id,
Price,
hist_lastupdated,
lag(hist_lastupdated) over (partition by product_id order by hist_lastupdated) as start_date,
hist_lastupdated as end_date
from the_table
You didn't explain where the start_date for the first column is calculated. If that is beginning of the month from hist_lastupdated you can do something like this:
lag(hist_lastupdated, 1, date_trunc('month', hist_lastupdated)) over (...)

Answer 2

I'm not sure how you would do this with just SQL but if you're able to do a bit of scripting you can write up a quick program that goes something like this (pseudocode):
lines = execute(SELECT product_id, price, hist_lastupdated FROM ProductTable)
startDate = 00:00:00 2014-05-01
outputLines = []
for row in lines:
outLine = []
outline.append(row[0])
outline.append(row[1])
outline.append(row[2])
outline.append(startDate)
outline.append(row[2])
startDate = row[2]
#Now do what you want with the output you have in a nice list of lists in the format you need, insert into a different table, write to a file, whatever you want.

Answer 3

I would use one of these solutions with MS SQL Server. Hopefully one of them will apply to your problem.
Pure SQL statement would look like this:
select
t.product_id, t.price, s.start_date, t.end_date
from
product t
outer apply
(
select top 1
end_date start_date
from
product o
where
o.end_date < t.end_date
order by
o.end_date desc
) s
The cross apply for each record returned can be a performance problem even with good indexing.
If your SQL server supports the LAG function:
select
t.product_id, t.price,
LAG(T.end_date) over (order by t.end_date),
t.end_date
from
product t
Or you may find a way to do the same thing with variables in an update statement to "remember" the value in the previously updated record like the T-SQL:
-- Insert the desired output into a table variable that also has a start_date field.
-- Be sure to insert the records ordered by the date value.
declare #output table (product_id int, price numeric(10,2), [start_date] datetime, [end_date] datetime)
insert #output (product_id, price, end_date)
select 1, 10, '1/1/2015'
union all select 2, 11, '2/1/2015'
union all select 3, 15, '3/1/2015'
union all select 4, 20, '4/1/2015'
order by 3
-- Update the start date using the end date from the previous record
declare #start_date datetime, #end_date datetime
update
#output
set
#start_date = #end_date,
#end_date = end_date,
start_date = #start_date
select * from #output
I don't think this technique is recommended by Microsoft, but it has served me well and worked consistently. I only used this technique with table variables. I would be less inclined to trust the update sequence of records in in an actual table. Now I would use LAG() instead.

Answer 4

This is the solution that i find it,i wanted to work with the lag function but the result is not what i wanted to have.
The solution :
WITH
price_table_1 as (
select
-1 + ROW_NUMBER() over (partition by t1.product_id,t1.id ,t1.channel_id) as rownum_w1,
t1.id,
t1.product_id,
t1.channel_id,
t1.member_id,
t1.quantity,
t1.price,
t1.promo_dt_start,
t1.promo_dt_end,
t1.hist_lastupdated
FROM dwh_prod.hist_prices t1
where t1.channel_id='1004' and t1.product_id = '5896' and t1.quantity = '1' and t1.promo_dt_start is null
order by t1.product_id,t1.channel_id,t1.hist_lastupdated
),price_table_2 as (
select
ROW_NUMBER() over (partition by t2.product_id,t2.id ,t2.channel_id) as rownum_w2,
t2.id,
t2.product_id,
t2.channel_id,
t2.member_id,
t2.quantity,
t2.price,
t2.promo_dt_start,
t2.promo_dt_end,
t2.hist_lastupdated
FROM dwh_prod.hist_prices t2
where t2.channel_id='1004' and t2.product_id = '5896' and t2.quantity = '1' and t2.promo_dt_start is null
order by t2.product_id,t2.channel_id,t2.hist_lastupdated
)
select
t1.id,
t1.product_id,
t1.channel_id,
t1.member_id,
t1.quantity,
t1.price,
t1.promo_dt_start,
t1.promo_dt_end,
t2.hist_lastupdated as start_date,
t1.hist_lastupdated as end_date
FROM price_table_1 t1
inner join price_table_2 t2
on t2.product_id = t1.product_id and t2.id = t1.id and t2.channel_id = t1.channel_id
and rownum_w1 = (rownum_w2)
UNION ALL
select
t1.id,
t1.product_id,
t1.channel_id,
t1.member_id,
t1.quantity,
t1.price,
t1.promo_dt_start,
t1.promo_dt_end,
CONVERT(TIMESTAMP,'2014-01-01') as start_date,
t1.hist_lastupdated as end_date
FROM price_table_1 t1
where rownum_w1 = '0';