SQL where 与 Union 和 Left答案

【问题标题】：SQL where with Union and LeftSQL where 与 Union 和 Left
【发布时间】：2018-04-13 10:30:58
【问题描述】：

是否可以在 Left Join 的 [ID] 中而不是在 [TypeID] 中添加条件？

 SQL = "SELECT " & _
          "ADate As NewDate, " & _
          "tblA.TypeID as ID, " & _
          "tblAB.TypeControl as ControlID " & _
          "FROM tblA " & _
               "LEFT OUTER JOIN tblAB " & _
               "ON tblAB.TypeID = tblA.TypeID " & _
               "WHERE tblA.TypeID = " & Counter & " " & _     => Delete this one.
       "UNION ALL SELECT " & _
          "BDate As NewDate, " & _
          "tblB.TypeID as ID, " & _
          "tblAB.TypeControl as ControlID " & _
          "FROM tblB " & _
               "LEFT OUTER JOIN tblAB " & _
               "ON tblAB.TypeID = tblB.TypeID " & _
               "WHERE tblB.TypeID = " & Counter & " " & _     => Delete this one.
       ===
       and place one WHERE on ID here
       "WHERE ID = " & Counter & " " & _    => Like this one. But I am getting an error.
       ===
       "ORDER BY NewDate;"

从 tblA 和 tblB 中删除两个 WHERE。最后在ID中添加一个。并创建这个。

 SQL = "SELECT " & _
          "ADate As NewDate, " & _
          "tblA.TypeID as ID, " & _
          "tblAB.TypeControl as ControlID " & _
          "FROM tblA " & _
               "LEFT OUTER JOIN tblAB " & _
               "ON tblAB.TypeID = tblA.TypeID " & _
       "UNION ALL SELECT " & _
          "BDate As NewDate, " & _
          "tblB.TypeID as ID, " & _
          "tblAB.TypeControl as ControlID " & _
          "FROM tblB " & _
               "LEFT OUTER JOIN tblAB " & _
               "ON tblAB.TypeID = tblB.TypeID " & _
               "WHERE tblB.TypeID = " & Counter & " " & _
       "WHERE ID = " & Counter & " " & _
       "ORDER BY NewDate;"

提前谢谢你。

【问题讨论】：

呃……你到底是什么意思？您想在联接中使用字段别名吗？因为这是不可能的，除非你使用子查询，而且你不应该仅仅为了创建别名而使用子查询。
现在我在 [TypeID] 中有两个 WHERE 条件，分别在 tblA 和 tblB 上。我想在 [ID] 上创建一个 WHERE 条件。我更新了我的帖子。
请编辑您的问题并显示格式整齐的 SQL 查询，而不是代码版本。然后显示样本数据和预期输出。

标签： sql ms-access criteria

【解决方案1】：

您可以通过将整个查询包装在子查询中来仅检查一次 ID，然后在外部查询中检查 ID，例如：

SELECT * FROM (
    SELECT " & _
      "ADate As NewDate, " & _
      "tblA.TypeID as ID, " & _
      "tblAB.TypeControl as ControlID " & _
      "FROM tblA " & _
           "LEFT OUTER JOIN tblAB " & _
           "ON tblAB.TypeID = tblA.TypeID " & _
    "UNION ALL SELECT " & _
      "BDate As NewDate, " & _
      "tblB.TypeID as ID, " & _
      "tblAB.TypeControl as ControlID " & _
      "FROM tblB " & _
           "LEFT OUTER JOIN tblAB " & _
           "ON tblAB.TypeID = tblB.TypeID "
) WHERE ID =  " & Counter & "

但是，根据数据库引擎对此的优化方式，可能需要更长的时间来执行。我建议您不要这样做，并按原样保留您的查询。

（另外，我把引号留给你，因为你的问题不应该有这些）。

【讨论】：