querying the value of the previous row

In response to Chris Velevitch :

I'm to write a query like:-

select
���� case when column_name1 <> value_of_previous(column_name1)
��������� then column_name1 end as column
��� ,column_name2
from table
ordered by column_name1, column_name2

Okay, with this table:

test=# select * from foo;
col1 | col2
------+------
1 | 1
1 | 2
1 | 3
2 | 4
2 | 5
2 | 6
2 | 7
3 | 8
4 | 9
5 | 10
(10 rows)

you can do:

test=# select
case when col1::text <> coalesce(lag::text,'NULL') then col1 else null end as col1,
col2
from (
select col1, lag(col1) over (range unbounded preceding ),
col2
from foo
order by col2
) foo order by col2;
col1 | col2
------+------
1 | 1
| 2
| 3
2 | 4
| 5
| 6
| 7
3 | 8
4 | 9
5 | 10
(10 rows)

How do I do this? (I'm using pg 7.4)

Unfortunately (for you), i'm using a window-function, in this case
lag(), new since 8.4. Your version 7.4 has reached End-of-Lifetime, so
i suggest you update to 8.4.

Regards, Andreas
--
Andreas Kretschmer
Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header)
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Chris Velevitch wrote:

I'm to write a query like:-

select
case when column_name1 <> value_of_previous(column_name1)
then column_name1 end as column
,column_name2
from table
ordered by column_name1, column_name2

in order to get:-

column | column_name2
--------------+--------------
value_1_c1 | value_1_c2
| value_2_c2
| value_3_c2
value_2_c1 | value_4_c2
| value_5_c2
| value_6_c2
value_3_c1 | value_7_c2
| value_8_c2
| value_9_c2

How do I do this? (I'm using pg 7.4)

what does 'previous' mean here? thats not a concept SQL really has.

anyways, that sort of report output is something you usually do via your
reporting code