querying the value of the previous row

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)

Chris
--
Chris Velevitch
Manager - Adobe Platform Users Group, Sydney
m: 0415 469 095
www.apugs.org.au

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