如何在 SQL Server 中的两个 SELECT 语句之间执行 LEFT JOIN？

Question

我在 SQL Server 中有两个如下所示的 SELECT 语句：

(SELECT [UserID] FROM [User])
(SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043)

我想在 [UserID] 属性和 [TailUser] 属性上的这两个 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以及缺失记录的 NULL 值连接起来。我该怎么做？

Answer 1

SELECT * FROM 
(SELECT [UserID] FROM [User]) a
LEFT JOIN (SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043) b
ON a.UserId = b.TailUser

Answer 2

SELECT [UserID] FROM [User] u LEFT JOIN (
SELECT [TailUser], [Weight] FROM [Edge] WHERE [HeadUser] = 5043) t on t.TailUser=u.USerID

Answer 3

select *
from user
left join edge
on user.userid = edge.tailuser
and edge.headuser = 5043

Answer 4

试试这个：

SELECT user.userID, edge.TailUser, edge.Weight 
FROM user
LEFT JOIN edge ON edge.HeadUser = User.UserID
WHERE edge.HeadUser=5043

OR

AND edge.HeadUser=5043

代替 WHERE 子句。