How to use NULL in SELECT statement in quotes

Hi,

I am enclosing a select statement inside quotes in a form.  At runtime, it gives error : Cannot create record group.  The same query works perfectly fine from outside say Toad.  I deleted some where conditions and zeroed in on this finding that the command 'r.mdle is NULL' is creating the problem.  How do I use the NULL command when using SELECT command inside quotes?

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 rg_id := Create_Group_From_Query( rg_name,
             'select distinct com_name, com_cmpnt_code '||
             'from cmpnt_mst c, mdle_to_cmpnt_mpng_mst m, reprts r '||
             'where c.com_cmpnt_code = m.mcm_cmpnt_code '||