如何在 SQL Server 中的两个 SELECT 语句之间执行 LEFT JOIN？
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How to perform a LEFT JOIN in SQL Server between two SELECT statements?
提问by moorara
I have two SELECT statements in SQL Server like these:
我在 SQL Server 中有两个 SELECT 语句，如下所示：
(SELECT [UserID] FROM [User])
(SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043)
I want to perform a LEFT JOIN between these two SELECT statements on [UserID] attribute and [TailUser] attribute. I want to join existent records in second query with the corresponding records in first query and NULL value for absent records. How can I do this?
我想在 [UserID] 属性和 [TailUser] 属性的这两个 SELECT 语句之间执行 LEFT JOIN。我想将第二个查询中的现有记录与第一个查询中的相应记录和缺失记录的 NULL 值连接起来。我怎样才能做到这一点？
回答by Derek Kromm
SELECT * FROM
(SELECT [UserID] FROM [User]) a
LEFT JOIN (SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043) b
ON a.UserId = b.TailUser
回答by Icarus
SELECT [UserID] FROM [User] u LEFT JOIN (
SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043) t on t.TailUser=u.USerID
回答by Icarus
select *
from user
left join edge
on user.userid = edge.tailuser
and edge.headuser = 5043
回答by deepcell
Try this:
尝试这个：
SELECT user.userID, edge.TailUser, edge.Weight
FROM user
LEFT JOIN edge ON edge.HeadUser = User.UserID
WHERE edge.HeadUser=5043
OR
或者
AND edge.HeadUser=5043
instead of WHERE clausule.
而不是 WHERE clausule。
