我在SO上看到过类似的错误，但我没有找到解决我的问题的方法。 我有一个SQL查询像:

SELECT DISTINCT
        a.maxa ,
        b.mahuyen ,
        a.tenxa ,
        b.tenhuyen ,
        ISNULL(dkcd.tong, 0) AS tongdkcd
FROM    phuongxa a ,
        quanhuyen b
        LEFT OUTER JOIN ( SELECT    maxa ,
                                    COUNT(*) AS tong
                          FROM      khaosat
                          WHERE     CONVERT(DATETIME, ngaylap, 103) BETWEEN 'Sep 1 2011'
                                                              AND
                                                              'Sep 5 2011'
                          GROUP BY  maxa
                        ) AS dkcd ON dkcd.maxa = a.maxa
WHERE   a.maxa <> '99'
        AND LEFT(a.maxa, 2) = b.mahuyen
ORDER BY maxa;

当我执行这个查询时，错误结果是: 无法绑定多部分标识符“a.maxa”。为什么? P/s:如果我把查询分成2个单独的查询，它运行正常。

SELECT DISTINCT
        a.maxa ,
        b.mahuyen ,
        a.tenxa ,
        b.tenhuyen
FROM    phuongxa a ,
        quanhuyen b
WHERE   a.maxa <> '99'
        AND LEFT(a.maxa, 2) = b.mahuyen
ORDER BY maxa;

and

SELECT  maxa ,
        COUNT(*) AS tong
FROM    khaosat
WHERE   CONVERT(DATETIME, ngaylap, 103) BETWEEN 'Sep 1 2011'
                                        AND     'Sep 5 2011'
GROUP BY maxa;