Mysql右外部连接无法正常工作100％

Question

我是mysql的新手，编写了My sql查询，并尝试用两个子查询进行外部联接，但是结果未在此处显示准确的结果

SELECT a.lpcode,a.lpname,a.companycode,a.zone,a.tdy_growr,a.tdy_acres,b.tdate_growr,tdate_acres,a.name
  FROM    (SELECT z.lpcode,
                  x.companycode,
                  z.lpname,
                  z.zone,
                  z.name,
                  count(x.vehicleno) tdy_growr,
                  sum(x.haulagecode) tdy_acres
             FROM gis.registration x, loadingpoint z
            WHERE x.date =
                     (SELECT max(a.date)
                        FROM gis.registration a
                       WHERE     a.fieldno > 0
                             AND a.haulagecode > 0
                             AND a.isaccepted = 1)
                  AND z.lpcode = x.lpcode
                  AND x.fieldno > 0
                  AND x.haulagecode > 0
                  AND x.isaccepted = 1
           GROUP BY x.lpcode) a
       RIGHT OUTER JOIN
          (SELECT r.lpcode,
                  count(r.vehicleno) tdate_growr,
                  sum(r.haulagecode) tdate_acres
             FROM gis.registration r, loadingpoint l
            WHERE r.fieldno > 0 AND r.haulagecode > 0 AND r.isaccepted = 1
            AND r.lpcode = l.lpcode
           GROUP BY l.lpcode) b
       ON a.lpcode = b.lpcode
ORDER BY a.zone, a.lpcode

可能需要任何帮助，在此先感谢您

Answer 1

右外部联接的性能很差，在很多情况下，您不能更改查询的结构以使用左外部联接来代替，这会带来更好的性能。 您的查询正在执行的是从b获取所有行，而不管a中是否有要连接的行。 重写的相同查询是：

SELECT             a.lpcode,a.lpname,a.companycode,a.zone,a.tdy_growr,a.tdy_acres,b.tdate_growr,a.tdate_acres,a.name
FROM (SELECT r.lpcode,
              count(r.vehicleno) tdate_growr,
              sum(r.haulagecode) tdate_acres
         FROM gis.registration r, loadingpoint l
        WHERE r.fieldno > 0 AND r.haulagecode > 0 AND r.isaccepted = 1
        AND r.lpcode = l.lpcode
       GROUP BY l.lpcode) b
   LEFT OUTER JOIN (SELECT z.lpcode,
              x.companycode,
              z.lpname,
              z.zone,
              z.name,
              count(x.vehicleno) tdy_growr,
              sum(x.haulagecode) tdy_acres
         FROM gis.registration x, loadingpoint z
        WHERE x.date =
                 (SELECT max(a.date)
                    FROM gis.registration a
                   WHERE     a.fieldno > 0
                         AND a.haulagecode > 0
                         AND a.isaccepted = 1)
              AND z.lpcode = x.lpcode
              AND x.fieldno > 0
              AND x.haulagecode > 0
              AND x.isaccepted = 1
       GROUP BY x.lpcode) a
   ON a.lpcode = b.lpcode
ORDER BY a.zone, a.lpcode

这也将给出b中的所有行，而不管a中是否有匹配的行要连接。

如果您的问题是在所有这些列中都得到NULL：“ a.lpcode，a.lpname，a.companycode，a.zone，a.tdy_growr，a.tdy_acres，a.tdate_acres，a.name”，请使用应该使用您已经在使用的结构，但使用LEFT OUTER JOIN代替，该结构可能在此列“ b.tdate_growr”中返回null