How to use concat with a specific column like a@b

vsql=> select * from KeyValue;
 key | value 
-----+-------
 a   |     1
 b   |     2
 c   |     3
(3 rows)

vsql=> select concat(key, value) from KeyValue;
 concat 
--------
 a1
 b2
 c3
(3 rows)

vsql=> select key || value from KeyValue;
 ?column? 
----------
 a1
 b2
 c3
(3 rows)