无法以您要求的格式进行有用的查询。当使用PIVOT时，结果中预期的栏数应该永远相同。在您显示数据的格式中，如果有新书或新作者，则会新增额外的栏。
下面的查询仅适用于指定的帐套和作者：

WITH
    users (aid, username)
    AS
        (SELECT 1, 'author1' FROM DUAL
         UNION ALL
         SELECT 2, 'author2' FROM DUAL),
    books (bid, name, aid)
    AS
        (SELECT 1, 'x', 1 FROM DUAL
         UNION ALL
         SELECT 2, 'y', 1 FROM DUAL
         UNION ALL
         SELECT 3, 'z', 2 FROM DUAL),
    location (loc_id, loc_name)
    AS
        (SELECT 1, 'UK' FROM DUAL
         UNION ALL
         SELECT 2, 'USA' FROM DUAL
         UNION ALL
         SELECT 3, 'Europe' FROM DUAL),
    UserAssign (u_UID,
                Aid,
                bid,
                loc_id,
                color,
                reviews)
    AS
        (SELECT 1, 1, 1, 1, 'y', 12 FROM DUAL
         UNION ALL
         SELECT 2, 1, 2, 1, 'r', 14 FROM DUAL
         UNION ALL
         SELECT 3, 2, 3, 1, 'y', 11 FROM DUAL
         UNION ALL
         SELECT 4, 1, 1, 2, 'y', 10 FROM DUAL
         UNION ALL
         SELECT 5, 2, 3, 2, 'y', 112 FROM DUAL)
SELECT l.loc_name, p.*
  FROM (  --authors
          SELECT loc_id,
                 u.username                                  AS colheader,
                 MIN (color) || ',' || AVG (ua1.reviews)     AS avg_reviews
            FROM userassign ua1 JOIN users u ON (ua1.aid = u.aid)
        GROUP BY loc_id, u.username
        UNION ALL
        --books
        SELECT loc_id, b.name, TO_CHAR (ua2.reviews) AS avg_reviews
          FROM userassign ua2 JOIN books b ON (ua2.bid = b.bid))
       PIVOT (MIN (avg_reviews)
             FOR colheader
             IN ('author1' AS author1,
                'x' AS x,
                'y' AS y,
                'author2' AS author2,
                'z' AS z)) p
       JOIN location l ON (p.loc_id = l.loc_id)
ORDER BY l.loc_name;

LOC_NAME       LOC_ID AUTHOR1    X     Y     AUTHOR2    Z
___________ _________ __________ _____ _____ __________ ______
UK                  1 r,13       12    14    y,11       11
USA                 2 y,10       10          y,112      112