Question

Query explanation

Asked by: aman0711

Hi Experts,

                one of the senior member here helped me out with a query from this question:

                http://www.experts-exchange.com/Database/Oracle/PL_SQL/Q_24867793.html#a25731039

              I just want to know how this query works and few other questions. 

SELECT id, LAST_VALUE(login_date) OVER (PARTITION BY id ORDER BY login_date), COUNT( * ) OVER ()
FROM siweb_login_history
WHERE TRUNC(login_date) BETWEEN TO_DATE('2009-11-02', 'yyyy-mm-dd')
                            AND  TO_DATE('2009-11-03', 'yyyy-mm-dd')
                                  
1:
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4:

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Asked On
2009-11-03 at 09:19:12ID24867911
Tags

Oracle

Topics

PL / SQL

,

Oracle 10.x

Participating Experts
1
Points
500
Comments
10

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Answers

 

by: sdstuberPosted on 2009-11-03 at 09:32:46ID: 25731355

LAST_VALUE(login_date) OVER (PARTITION BY id ORDER BY login_date)

the "over" clause creates a window of rows.  the "partition by" clause groups the rows by ID and sorts them by login_date  so each window will only have data for a single id.

then LAST_VALUE  simply returns whatever is the last login_date within that window, which will be the most recent value

the COUNT(*) OVER()  does the same thing but because the OVER() clause is empty, the window spans the entire result set

 

by: aman0711Posted on 2009-11-03 at 09:38:58ID: 25731422

Thank you Sir.
 
  so this OVER keyword is part of Oracle analytics? thats what I got when I googled.

One last question:

Attached snapshot is the output from that query. So its showing every 3 records.

The actual requirements is something like,

Group the IDs
Show the most recent login_time and the count betweent he date range

sp for every ID, just one Row, most recent login time (The 3rd row in the snapshot) and the count.

Also, why the column headers have names of the syntax? Can we make those little simpler, something like ID, LOGIN_TIME and COUNT?

 

by: aman0711Posted on 2009-11-03 at 09:39:22ID: 25731428

Oops forgot to attach the snapshot

 

by: sdstuberPosted on 2009-11-03 at 09:46:11ID: 25731498

try this...

note, you can alias a column just by adding the name you want after the value or function you select

SELECT id, login_date, cnt
FROM (SELECT id,
login_date,
ROW_NUMBER() OVER (PARTITION BY id ORDER BY login_date DESC) rn,
COUNT( * ) OVER (PARTITION BY id) cnt
FROM siweb_login_history
WHERE TRUNC(login_date) BETWEEN TO_DATE('2009-11-02', 'yyyy-mm-dd')
AND TO_DATE('2009-11-03', 'yyyy-mm-dd'))
WHERE rn = 1

 

by: sdstuberPosted on 2009-11-03 at 09:48:49ID: 25731518

note, I've switched from LAST_VALUE to ROW_NUMBER

I could have also used...

WHERE login_date = last_login_date  

where last_login_date  is the aliased result of last_value, but row number if more intuitive to me, and it eliminates duplicates if you happend to have two logins for the same id within the same second.

 

by: aman0711Posted on 2009-11-03 at 09:57:07ID: 25731613

WOW.. perfect solution Sir.

Can I keep reading on this solution and close this question a little later, in case I have some more question?

So with your second query, My resultSet would have 3 columns ultimately named:

ID, LOGIN_DATE, CNT

 

by: sdstuberPosted on 2009-11-03 at 10:05:51ID: 25731691

yes and yes, but you can change them by adding/changing aliases

 

by: aman0711Posted on 2009-11-03 at 10:07:24ID: 25731704

Thanks a lot Sir :)

This really helped and a perfect solution..

 

by: aman0711Posted on 2009-11-04 at 09:42:02ID: 31649518

Thanks a lot Sir.

you are always great help for me. :)

 

by: sdstuberPosted on 2009-11-04 at 09:43:34ID: 25741834

glad I could help

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