使用额外变量的 Linq 多重连接条件

Question

我有这个查询：

SELECT *
FROM transaction t
JOIN transactionDetail toTrans ON t.id = toTrans.tId and toTrans.FlowDirection= 1
JOIN transactionDetail fromTrans ON t.id = fromTrans.tId and fromTrans.FlowDirection= 0

我尝试使用匿名类型重新创建，如 here 所述。

byte toFlow = 1;
byte fromFlow = 1;

from trans in data.Transactions
join toTrans in data.TransactionDetails on new {trans.id, toFlow} equals new {toTrans.tId, toTrans.FlowDirection}
join fromTrans in data.TransactionDetails on new { trans.id, fromFlow } equals new { fromTrans.tId, fromTrans.FlowDirection }

Flowdirection 始终为 1 或 0，因此我使用的是 toFlow 字节。然而，这给出了错误：

The type of one of the expressions in the join clause is incorrect.

根据this回答，名称和类型都需要匹配。这意味着：

byte FlowDirection= 1;

from trans in data.Transactions
join toTrans in data.TransactionDetails on new {trans.id, FlowDirection} equals new {toTrans.tId, toTrans.FlowDirection}
join fromTrans in data.TransactionDetails on new { trans.id, FlowDirection} equals new { fromTrans.tId, fromTrans.FlowDirection }

哪个有效！但是，第二个连接需要 FlowDirection 的值为 0 而不是 1。如何更改 FlowDirection 的值？我不能更改变量的名称或在匿名对象中减去 1，否则这很容易。

Answer 1

How can I change the value of FlowDirection? I can't change the name of the variable or subtract 1 inside the anonymous object

要更改匿名对象中的变量，只需执行以下操作：

new { fromTrans.tId, FlowDirection = fromTrans.FlowDirection - 1 }

例如。

Answer 2

只是为了扩展评论：

Surely you can just use two constants (or literals)?, i.e.

from trans in data.Transactions
join toTrans in data.TransactionDetails 
  on new {ID = trans.id, Flow = (byte)1} 
  equals new {Id = toTrans.tId, Flow = toTrans.FlowDirection}
join fromTrans in data.TransactionDetails 
  on new { Id = trans.id, Flow = (byte)0} 
  equals new { Id = fromTrans.tId, Flow = fromTrans.FlowDirection }

Could FlowDirect - 1 not work because it turns FlowDirect into an int instead? Does subtracting an int from a byte turn the byte into an int maybe? Otherwise, I really don't know why your code works.

是的，您需要将结果转换回字节（或将文字 1 转换为 byte，以便使用字节运算符“-”）

Answer 3

另一个想法：

from trans in data.Transactions
join toTrans in data.TransactionDetails on trans.id equals new toTrans.tId
join fromTrans in data.TransactionDetails on trans.id equals fromTrans.tId
where toTrans.FlowDirection == 1 && fromTrans.FlowDirection == 1

我认为这个选项应该更容易阅读。

使用额外变量的 Linq 多重连接条件

Linq multiple join conditions using extra variables

c#

linq