SELECT within the FROM clause?

From: Dereck L. Dietz <dietzdl_at_ameritech.net>
Date: Sat, 09 Sep 2006 20:22:51 GMT
Message-ID: <voFMg.3661$MF1.3024@newssvr25.news.prodigy.net>

I have never seen code where there is a SELECT within the FROM clause like below. I have always seen it in the WHERE clause. Can anybody explain what a SELECT in this location is doing?

P.S.: This is not my code - I'm just trying to figure it out and maybe tune it if I can after I can understand what it's doing.

FROM CLAIM.CLM_RX A, (SELECT DISTINCT

                                                A.AHFS_THPY_CD,
                                                A.TYPE_1,
                                                NULL AS TYPE_2,
                                                A.SPFC_THPY_CLASS_CD,
                                                B.NATL_DRUG_CD AS NDC,
                                                A.HMP_DISEASE_CODE
                                                FROM
bcn_hmp.ref_hmp_rx_class A,
                                                RPT.DB2_NDC B
                                                WHERE
A.AHFS_THPY_CD=B.AHFS_THPY_CD
                                                AND A.HMP_DISEASE_CODE

IN('LL','ST')) B
WHERE A.NATL_DRUG_CD = B.NDC
AND A.PCH_DATE BETWEEN TO_DATE('&BDATE','YYYY-MM-DD') AND TO_DATE('&EDATE','YYYY-MM-DD') Received on Sat Sep 09 2006 - 15:22:51 CDT