Since you send a string to SQLServer, I don't see why you could not do this:

mcproj="'000609','000172','001113'"

? mcproj
? SQLEXEC(1,"SELECT *;
 FROM ZAKI.PROJ Proj;
 WHERE {fn LEFT(Proj.PROJ_ID,6)} IN (" + mcproj + ") AND {fn LENGTH(Proj.PROJ_ID)}=10;
 ORDER BY Proj.PROJ_ID")

>Can anyone tell me why the first SQL statement returns 17 records and the second returns none? Both SQLEXEC()'s return 1 so there are no errors in either SELECT.