救急！奇怪的join和聚合函数的应用

searcher

免费乐捐，感谢支持

先看看sql 语句：

    select *
    from
    (
        select CompanyID,  Max(NewsDate) as LastestNewsDate
        from news
        group by CompanyID
    )c
    LEFT JOIN
    (
        select SecurityID, Max(PriceDate) as LastestPriceDate
        from stock_data
        group by SecurityID
    )b
    ON b.SecurityID = c.CompanyID
=========================================================
现象如下：执行的结果中LastestPriceDate 总是和LastestNewsDate完全一样（当然实际table中的数据不一样），感觉就像是copy的。感觉极其想不通，跟踪了一下query plan(DBArtisan 8.1)，结果：

QUERY PLAN FOR STATEMENT 1 (at line 1).


    STEP 1
        The type of query is SELECT (into Worktable1).
        GROUP BY
        Evaluate Grouped MAXIMUM AGGREGATE.

        FROM TABLE
            news
        Nested iteration.
        Table Scan.
        Forward scan.
        Positioning at start of table.
        Using I/O Size 8 Kbytes for data pages.
        With LRU Buffer Replacement Strategy for data pages.
        TO TABLE
            Worktable1.

    STEP 2
        The type of query is SELECT.

        FROM TABLE
            Worktable1.
        Nested iteration.
        Table Scan.
        Forward scan.
        Positioning at start of table.
        Using I/O Size 8 Kbytes for data pages.
        With MRU Buffer Replacement Strategy for data pages.

        FROM TABLE
            Worktable1.
        Nested iteration.
        Using Clustered Index.
        Forward scan.
        Positioning by key.
        Using I/O Size 8 Kbytes for data pages.
        With MRU Buffer Replacement Strategy for data pages.
感觉
   select SecurityID, Max(PriceDate) as LastestPriceDate
        from stock_data
        group by SecurityID中的聚合函数根本就没有执行。

请高手指教，[em03]