To: pgsql-sql@postgresql.org
From: Harald Fuchs <hari.fuchs@gmail.com>
Subject: Re: Display group title only at the first record within each group
Date: Wed, 24 Aug 2016 10:03:27 +0200
Lines: 42
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CN <cnliou9@fastmail.fm> writes:

> Hi!
>
> Such layout is commonly seen on real world reports where duplicated
> group titles are discarded except for the first one.
>
> CREATE TABLE x(name TEXT,dt DATE,amount INTEGER);
>
> COPY x FROM stdin;
> john    2016-8-20       80
> mary    2016-8-17       20
> john    2016-7-8        30
> john    2016-8-19       40
> mary    2016-8-17       30
> john    2016-7-8        50
> \.
>
> My desired result follows:
>
> john    2016-07-08      50
> 					30
> 		2016-08-19      40
> 		2016-08-20      80
> mary    2016-08-17      20
> 					30

Use window functions:

SELECT CASE
       WHEN lag(name) OVER (PARTITION BY name ORDER BY name, dt) IS NULL
       THEN name
       ELSE NULL
       END,
       CASE
       WHEN lag(dt) OVER (PARTITION BY name, dt ORDER BY name, dt) IS NULL
       THEN dt
       ELSE NULL
       END,
       amount
FROM x
ORDER BY name, dt


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