To: pgsql-sql@postgresql.org
From: Harald Fuchs <hari.fuchs@gmail.com>
Subject: Re: Issues with lag command
Date: Sat, 29 Jul 2017 10:05:52 +0200
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Igor Neyman <ineyman@perceptron.com> writes:

> Hello
> I have a test table with the following structure (2 columns: ID and time_id )and data
>
> ID, time_id
> 1;"2015-01-01"
> 2;""
> 3;""
> 4;"2015-01-02"
> 5;""
> 6;""
> 7;""
> 8;"2015-01-03"
> 9;""
> 10;""
> 11;""
> 12;""
> 13;"2015-01-05"
> 14;""
> 15;""
> 16;""
> I'd like to update line 2 and 3 with the date in record 1 (2015-01-01)
> Update line 5,6 and 7 with the date in record 4 (2015-01-02) and so on
> How about simple SQL instead of PlSql:
>
> UPDATE test T1 SET time_id = (SELECT T2.time_id FROM test T2 WHERE T2.id =
>           (SELECT max(T3.id) FROM test T3 WHERE T3.id < T1.id AND T3.time_id IS NOT NULL)
> )
>    WHERE T1.time_id IS NULL;

You don't need that many table aliases:

UPDATE test
SET time_id = 
       ( SELECT T1.time_id
         FROM test T1
         WHERE T1.id < test.id
           AND T1.time_id IS NOT NULL
           ORDER BY T1.id DESC
           LIMIT 1
       )
WHERE time_id IS NULL;


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