Constant and SQL (in SQLRPG)

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There seems to be a problem using a constant into an SQL statement in a
SQLRPG program. The easy solution is to use a variable instead of a
constant. But I'd like to know if there is a way to use a constant and
if not, why this is not possible.

Here is an example of what I've tried and doesn't seem to work:

d cst             c                   const('001')
c/EXEC SQL
c+  SELECT ...
c+        WHERE zone1 = :cst
c/END-EXEC

Instead, if I declare cst as a variable, it works:
d cst             s            3a       inz('001')

Thanks.

Michel Delisle