Question

我的查询结果集大约是6万行：

SELECT f.client_id,
       f.SECTION_A,
       Month(f.received_date) Month,
       Count(*)               Count
FROM   MILLENNIUM_DW_DEV..F_ACCESSION_DAILY f
       LEFT JOIN salesdwh..TestPractices t
         ON F.CLIENT_ID = t.ClientID
WHERE  t.ClientID IS NULL
       AND Month(f.received_date) BETWEEN 1 AND 11
       AND Year(f.received_date) = 2012
GROUP  BY f.SECTION_A,
          Month(f.received_date),
          f.client_id

然而，当我添加2个左连接时，它将它限制为56k行！

SELECT f.client_id,
       f.SECTION_A,
       Month(f.received_date) Month,
       Count(*)               Count,
       s.SALES_REP_NAME,
       s.REGION_NAME
FROM   MILLENNIUM_DW_DEV..F_ACCESSION_DAILY f
       LEFT JOIN salesdwh..TestPractices t
         ON F.CLIENT_ID = t.ClientID
       LEFT JOIN MILLENNIUM_DW_DEV..D_CLIENT d
         ON d.CLIENT_ID = f.CLIENT_ID
       LEFT JOIN MILLENNIUM_DW_DEV..D_SALES_HIERARCHY s
         ON s.SUB_TERRITORY_NBR = d.SUB_TERRITORY_NBR
WHERE  t.ClientID IS NULL
       AND Month(f.received_date) BETWEEN 1 AND 11
       AND Year(f.received_date) = 2012
       AND d.REC_ACTIVE_FLG = 1
       AND s.REC_ACTIVE_FLG = 1
GROUP  BY f.SECTION_A,
          Month(f.received_date),
          f.client_id,
          s.SALES_REP_NAME,
          s.REGION_NAME

为什么我的左手会加入LIMIT我的结果集？

Answer 1

您不仅要添加左外连接，还要添加过滤条件：

and d.REC_ACTIVE_FLG=1
and s.REC_ACTIVE_FLG=1

这有效地将您的外连接转换为内连接。它还可能过滤掉d和/或s存在的一些记录，但REC_ACTIVE_FLG不等于1。

Answer 2

所以修复是将条件添加到on子句而不是where子句中：

FROM   MILLENNIUM_DW_DEV..F_ACCESSION_DAILY f
       LEFT JOIN salesdwh..TestPractices t
         ON F.CLIENT_ID = t.ClientID
       LEFT JOIN MILLENNIUM_DW_DEV..D_CLIENT d
         ON d.CLIENT_ID = f.CLIENT_ID and d.REC_ACTIVE_FLAG = 1
       LEFT JOIN MILLENNIUM_DW_DEV..D_SALES_HIERARCHY s
         ON s.SUB_TERRITORY_NBR = d.SUB_TERRITORY_NBR and s.REC_ACTIVE_FLAG = 1

为什么左联接限制我的结果集？

2 个答案: