PARTITION BY(Oracle)

社員テーブル

CREATE TABLE MEMBER (
    MEMBER_ID		NUMBER(3,0),
    MEMBER_NAME		VARCHAR2(50)
    SECTION_ID		NUMBER(3,0),
);

SECTION_IDでGROUP BYした結果をMEMBER_IDで並び替えて、先頭の1件を取得

SELECT
	MEMBER_ID,
	MEMBER_NAME
FROM (
    SELECT 
	MEMBER_ID,
	MEMBER_NAME,
	ROW_NUMBER() OVER (PARTITION BY SECTION_ID ORDER BY MEMBER_ID) RNUM
     FROM
	MEMBER
)
WHERE
    RNUM = 1

参考:

http://www.shift-the-oracle.com/sql/functions/row_number-practice.html