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 SELECT 语句，如下所示：

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

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？

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

要么

AND edge.HeadUser=5043

而不是 WHERE 子句。