Question

I have a table in PostgreSQL DB like this:

 Client | Rate | StartDate|EndDate     
 A      | 1000 | 2005-1-1 |2005-12-31
 A      | 2000 | 2006-1-1 |2006-12-31
 A      | 3000 | 2007-1-1 |2007-12-31  
 B      | 5000 | 2006-1-1 |2006-12-31  
 B      | 8000 | 2008-1-1 |2008-12-31  
 C      | 2000 | 2006-1-1 |2006-12-31  

I want to get the latest change, like this table. How?

 Client | Rate | StartDate|EndDate    |Pre Rate | Pre StartDate |Pre EndDate    
 A      | 3000 | 2007-1-1 |2007-12-31 | 2000    | 2006-1-1      |2006-12-31  
 B      | 8000 | 2008-1-1 |2008-12-31 | 5000    | 2006-1-1      |2006-12-31   
 C      | 2000 | 2006-1-1 |2006-12-31 

Answer 1

I can’t help thinking there’s a simpler way to express this.

with current_start_dates as (
  select client, max(startdate) cur_startdate
  from client_rates
  group by client
),
extended_client_rates as (
  select client, rate, startdate, enddate, 
    lag(rate, 1) over (partition by client order by startdate) prev_rate,
    lag(startdate,1) over (partition by client order by startdate) prev_startdate,
    lag(enddate,1) over (partition by client order by startdate) prev_enddate
  from client_rates
)
select cr.* 
from extended_client_rates cr
inner join current_start_dates csd on csd.client = cr.client 
                                  and csd.cur_startdate = cr.startdate