ORA-00936 error from SQL expression in SQL*Loader script

Question

I am getting the above error on the following line in my SQL*Loader script:
DIA_CLM_RES_OID DECIMAL EXTERNAL
"SELECT N_ORG_ENTY_ID FROM TESTG4.ORG_ENTITY
WHERE N_USER_ID =
(SELECT UNIQUE WSR_NT_ID FROM CONV_CLM_RESOURCE
WHERE CLM_RES_OID = :DIA_CLM_RES_OID)",
What I am basically trying to do is a 2-table lookup of a value:
1. Find a row in table CONV_CLM_RESOURCE where the value in column CLM_RES_OID matches the value in the input file in field DIA_CLM_RES_OID.
2. Take the value of field WSR_NT_ID from that row and use it to find a row in table TESTG4.ORG_ENTITY.
3. Take the value of field WSR_NT_ID from that row and set it in the target table in field DIA_CLM_RES_OID.
In other words, I am essentially trying to translate the input value by using two other tables to lookup the value to translate to. However, no matter how I arrange it, I keep getting the "ORA-00936: missing expression" error on this statement.
Can anyone see what I am doing wrong, or perhaps suggest a better way of accomplishing a two-table translation of a value?
Thanks!

Answer 1

Still not sure why this doesn't work, but I was able to create and use a function to do this instead, which is probably a better approach anyway.