CODEWITHSUNDEEP
c#linq
yesman
yesman

Reputation: 7838

Linq multiple join conditions using extra variables

I have this query:

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

Which I tried to recreate using anonymous types, as explained here.

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 is always either 1 or 0, so I'm using the toFlow byte. This however, gives the error:

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

According to this answer, both name and types need to match. Which would mean:

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 }

Which works! However, the second join needs to have a FlowDirection of value 0 instead of 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, or else this would have been easy.

Upvotes: 5

Views: 2058

Answers (3)

Andrew
Andrew

Reputation: 7880

Another idea:

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

I think this option should be easier to read.

Upvotes: 0

StuartLC
StuartLC

Reputation: 107387

Just to expand on the commentary:

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.

Yes, you would need to cast the result back to byte (or the literal 1 to byte so that byte operator "-" is used)

Upvotes: 4

dav_i
dav_i

Reputation: 28157

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

To change your variable inside the anonymous object simply do:

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

for example.

Upvotes: 0

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