SQL : problem with if statement

Hi,
I would like to know if there is any possibility to code in PSQL something like this (from Sybase Transact SQL):

IF EXISTS (SELECT k_num FROM kunde)
BEGIN
...
END which tests for the existece of a certain row. This is faster than counting the number of rows because EXISTS stops the select after having found the first row.

Thank you for any hints,
Astrid.

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Astrid Ritscher, ISL, Bremen, Germany