Обсуждение: Please help me write a query

Say I have a table that stores state transitions over time like so:

Lost me a bit, do you mean DISTINCT?

select distinct state1, first(timestamp) from table????

On May 27, 2010, at 10:04 AM, Nikolas Everett wrote:

> Say I have a table that stores state transitions over time like so:
> id, transitionable_id, state1, state2, timestamp
>
> I'm trying to write a query that coalesces changes in state2 away to produce just a list of transitions of state1.  I
guessit would look something like 
>
> SELECT state1, FIRST(timestamp)
> FROM table
>
> but I have no idea how to aggregate just the repeated state1 rows.

Sorry.  Here is the setup:

On 5/27/2010 9:04 AM, Nikolas Everett wrote:
> Say I have a table that stores state transitions over time like so:
> id, transitionable_id, state1, state2, timestamp
>
> I'm trying to write a query that coalesces changes in state2 away to
> produce just a list of transitions of state1.  I guess it would look
> something like
>
> SELECT state1, FIRST(timestamp)
> FROM table
>
> but I have no idea how to aggregate just the repeated state1 rows.

if i understand what your  after

Select distinct transitinable_id, state1,  min(timestamp) from table
group by  transitinable_id, state1















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On 5/27/2010 9:45 AM, Nikolas Everett wrote:

Sorry.  Here is the setup:

CREATE TABLE test (id BIGSERIAL PRIMARY KEY, state1 INT NOT NULL, state2 INT NOT NULL, timestamp TIMESTAMP);

INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval '12 hours');

INSERT INTO test (state1, state2, timestamp) VALUES (1, 2, now() - interval '11 hours');

INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval '10 hours');

INSERT INTO test (state1, state2, timestamp) VALUES (2, 1, now() - interval '9 hours');

INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval '8 hours');

I want to write a query that spits out:

 state1 |         timestamp          

--------+----------------------------

      1 | now() - interval '12 hours'

      2 | now() - interval '9 hours'

      1 | now() - interval '8 hours'

Have a question what makes  these values different other than the timestamp???

1, 1, now() - interval '12 hours'
1, 1, now() - interval '10 hours'

The reason i ask, is because you show 1, 1, now() - interval '8 hours'  in the desired output.   What logic keeps the 8 hour and 12 hour but not the 10hour interval???

Its kinda hard to understand why the 10hour interval is being skipped???

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Thank you.

The 10 and 11 hour interval are being skipped because I'm only interested in the transitions of state 1. State 1 only transitioned three times at now - 12, now - 9 and now - 8.

Nikolas Everett <nik9000@gmail.com> wrote:

> Sorry.  Here is the setup:
> CREATE TABLE test (id BIGSERIAL PRIMARY KEY, state1 INT NOT NULL, state2 INT
> NOT NULL, timestamp TIMESTAMP);
> INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval
> '12 hours');
> INSERT INTO test (state1, state2, timestamp) VALUES (1, 2, now() - interval
> '11 hours');
> INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval
> '10 hours');
> INSERT INTO test (state1, state2, timestamp) VALUES (2, 1, now() - interval
> '9 hours');
> INSERT INTO test (state1, state2, timestamp) VALUES (1, 1, now() - interval
> '8 hours');


> I want to write a query that spits out:
>  state1 |         timestamp
> --------+----------------------------
>       1 | now() - interval '12 hours'
>       2 | now() - interval '9 hours'
>       1 | now() - interval '8 hours'

> Standard grouping destroys the third row so that's out.  No grouping at all
> gives repeats of state1.  Is this what partitioning is for?

Partitioning usually means splitting data across several
tables for faster access which is probably not what you want
here.

  A simple solution would be to use LAG() and discard rows
where the current value is equal to the preceding value:

| SELECT state1, timestamp
|   FROM
|     (SELECT id,
|             state1,
|             state2,
|             LAG(state1) OVER (ORDER BY timestamp) AS prevstate1,
|             timestamp FROM test) AS SubQuery
|   WHERE state1 IS DISTINCT FROM prevstate1
|   ORDER BY timestamp;

Tim