Question

A small disaster happened to me on my database postgresql ver. 8.4.
Help me please!

I have to follow my visitors on my webpage in two tables like:

TABLE history

whereseenlast   varchar(50)
uniqid          varchar(13) primary key
whenseenlast    timestamp
bla1            varchar(10)
bla2            varchar(10)
bla3            varchar(10)

example:

where1.html, **1**, 2000-01-01 00:00:00, somebla1, somebla2, somebla3
where2.html, **1**, 2005-05-05 05:06:07, somebla1, somebla2, somebla3
...
where12345.html, **1**, 2012-11-22 11:22:33, somebla1, somebla2, somebla3

TABLE visitors (uniqid is primary key, each visitor can be found just once in this table)

wherseenfirst   varchar(50)
whereseenlast   varchar(50)
whenseenfirst   timestamp
whenseenlast    timestamp
uniqid          varchar(13) PRIMARY KEY
notes           varchar (20)
xyz             varchar(20)

example:

where1.html, where12345.html, 2000-01-01 00:00:00,  2012-11-22 11:22:33, 1, somenote, somexyz 

I accidentally deleted the TABLE visitors and I have no backup. But, I am sure it is easy for an expert to rebuild the TABLE visitors from the TABLE history with a query.

(I know I will not be able to restore the notes and xyz COLUMNS from TABLE history but that is not a big deal)

Could you please suggest solution?

Answer 1

Is this your home assignment for database class?

You first have to select the latest and earliest rows from History for every visitor:

select uniqid, max(whenseenlast) from history group by uniqid

and

select uniqid, min(whenseenlast) from history group by uniqid

then select the records that match them for your history table, something like this:

select frst.uniqid, whenseenfirst, whereseenfirst, whenseenlast, whereseenlast
from
(select h.uniqid, mn.mnseen as whenseenfirst, h.whereseenlast as whereseenfirst
    from history h
    join (select uniqid, min(whenseenlast) as mnseen from history group by uniqid) mn
    on mn.mnseen = h.whenseenlast and mn.uniqid = h.uniqid) frst,
(select h.uniqid, mx.mxseen as whenseenlast, h.whereseenlast as whereseenlast
    from history h
    join (select uniqid, max(whenseenlast) as mxseen from history group by uniqid) mx
    on mx.mxseen = h.whenseenlast and mx.uniqid = h.uniqid) lst
where frst.uniqid = lst.uniqid

You can verify it on SQLFiddle