Oracle、カーソルを開き、多数の列の 1 つを変数に選択する方法

これならできると思います:

    curid NUMBER;
    desctab DBMS_SQL.DESC_TAB;
    colcnt NUMBER; -- total number of columns
    res NUMBER;
    V_ASN_COUNT NUMBER;
BEGIN

    OPEN O_CURSOR FOR            
    SELECT 
        column1, -- a bunch of columns
        column2,
        ...
        COUNT(DISTINCT SI.ASN_NO) OVER (PARTITION BY SI.ASN_NO) AS ASN_COUNT                                 
    FROM AN_ORDER_INFO OI, AN_SHIPMENT_INFO SI
    WHERE -- a bunch of criteria    

    curid := DBMS_SQL.TO_CURSOR_NUMBER (O_CURSOR);

    DBMS_SQL.DESCRIBE_COLUMNS(curid, colcnt, desctab);
    -- "ASN_COUNT" is the last column, i. e. "colcnt" refers to column number of "ASN_COUNT"
    -- or set colcnt directly, e.g. colcnt := 12;

    FOR i IN 1..colcnt LOOP
        IF desctab(i).col_type = 2 THEN
            DBMS_SQL.DEFINE_COLUMN(curid, i, V_ASN_COUNT);
        ELSIF desctab(i).col_type = 12 THEN
            DBMS_SQL.DEFINE_COLUMN(curid, i, datevar);
            .......
        ELSE
            DBMS_SQL.DEFINE_COLUMN(curid, i, namevar, 25);
        END IF;         
    END LOOP;
    -- I do not know if this loop is needed, perhaps you can simply do 
    -- DBMS_SQL.DEFINE_COLUMN(curid, colcnt, V_ASN_COUNT);
    -- for a single column

    res := DBMS_SQL.FETCH_ROWS(curid); -- Fetch only the first row, no loop required
    DBMS_SQL.COLUMN_VALUE(curid, colcnt, V_ASN_COUNT); -- Loop over all column not required, you just like to get the last column

    IF V_ASN_COUNT > 1 THEN
      RAISE MULTIPLE_ASNS;
    END IF;
    DBMS_SQL.CLOSE_CURSOR(curid);
  

 詳細については、Oracle ドキュメントを確認してください:DBMS_SQL.TO_CURSOR_NUMBER 
 関数。 
 

 ただし、カーソルのオープン/巻き戻しの問題は残ります!