I am not certain exactly what you are trying to do here.
If you need to do a left outer join which is typically done between data tables, I suggest you use the database expert and create a command object. The command object is really a place for the SQL query which can include the LEFT OUTER JOIN.
If you simply want to join the column value to to = SEP here is an example
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ToText({Credit.Customer Credit ID}) & "= Sep"
returns something like
"1234 = Sep"
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Hope this helps.