如果行为空，如何使用第二个 "ON" 条件将 table 与自身连接

Question

我有一个用户 table。其中，它有以下 4 列：

------ UserID ------ | ---- Username ----- | --- CreatedBy --- | ParentUserID

(PK, bigint, not null) | （字符（20），不为空） | (varchar(50), null) | (bigint, null)

ParentUserID 和 CreatedBy 分别通过 UserID 或 Username 指向 "owner account"。两者都是独一无二的。

CreatedBy 实际上从未 null，但 UserID 已编入索引，因此首选 ParentUserID - 这也是我们正在朝着的目标。

显然我并不流利SQL，但这是我的想法：

SELECT Users.*
  FROM tblUsers AS Owners
    LEFT JOIN tblUsers AS Users
      ON
        ISNULL(Users.ParentUserID = Owners.UserID,
          Users.CreatedBy = Owners.Username)
    WHERE Owners.UserID = 14;

据我所知：

SELECT ISNULL(POwners.UserID, COwners.UserID) AS OwnerID, Users.*
  FROM tblUsers AS Users
    RIGHT JOIN tblUsers AS POwners ON Users.ParentUserID = POwners.UserID
    RIGHT JOIN tblUsers AS COwners ON Users.CreatedBy = COwners.Username
WHERE OwnerID = 14;

虽然显然这行不通。在次要说明中，我还需要将其转换为 LINQ，但对于这个问题，到目前为止只有查询可以转换才相关，这是我期望的绝大多数查询。

Answer 1

join 条件只是布尔测试，因此您需要编写适当的布尔条件，例如(P or Q) AND R。你不能用 , 链接它们，所以...

... ON ISNULL((Users.ParentUserID = Owners.UserID) AND (Users.CreatedBy = Owners.Username))

或者任何你需要的逻辑。使其成为 VALID 布尔表达式是关键部分。

Answer 2

这似乎是对我有用的查询：

SELECT Users.*
  FROM tblUsers AS Owners
    LEFT JOIN tblUsers AS Users
      ON Users.ParentUserID = Owners.ParentUserID
        OR Users.CreatedBy = Owners.Username
  WHERE Owners.UserID = 14;

感谢的帮助。

至于LINQ，原来是only equijoins are supported，所以我做了这样的交叉连接：

from u in dbContext.tblUsers
from o in dbContext.tblUsers
where (u.ParentUserID == o.UserID || u.CreatedBy == o.Username)
    && o.UserID == 14
select u;

这变成了以下查询：

SELECT Users.*
    FROM  tblUsers AS Users
        CROSS JOIN tblUsers AS Owners
    WHERE(Users.ParentUserID = Owners.UserID OR Users.CreatedBy = Owners.Username)
        AND(Owners.UserID = 14)

如果行为空，如何使用第二个 "ON" 条件将 table 与自身连接

How to join table with itself, with a second "ON" criteria, if a row is null

sql-server

join

self-join