I imagine this forum posting, which I quote fully below, should answer the question.
Inside a procedure, function, or trigger definition, or in a dynamic SQL statement (embedded in a host program):
BEGIN ATOMIC
DECLARE example VARCHAR(15) ;
SET example = 'welcome' ;
SELECT *
FROM tablename
WHERE column1 = example ;
END
or (in any environment):
WITH t(example) AS (VALUES('welcome'))
SELECT *
FROM tablename, t
WHERE column1 = example
or (although this is probably not what you want, since the variable needs to be created just once, but can be used thereafter by everybody although its content will be private on a per-user basis):
CREATE VARIABLE example VARCHAR(15) ;
SET example = 'welcome' ;
SELECT *
FROM tablename
WHERE column1 = example ;