t-sql join help

Table_A

TxID    RelatedTxID
-------------------
1       NULL
2       NULL
3       1

Table_B

OrderID    TxID    OrderDescription
-----------------------------------
1    开发者_JAVA百科       1      Description_1
2           2      Description_2

I want to get an output which will give me order description for the transaction. But if the transaction does not have an order description I want to display it's related transaction's order description (Related transaction will always have an order description)

Output

TxID   RelatedTxID    OrderDescription
------------------------------------
1       NULL        Description_1
2       NULL        Description_2
3       1           Description_1

I am thinking of something like below but stuck at what should come in the ISNULL expression.

select 
a.TxID, 
a.RelatedTxID, 
ISNULL(b.OrderDescription, <<get its related transaction's order description>>)
from Table_A a
left outer join Table_B b
on a.TxID = b.TxID

Thanks

---

select 
    a.TxID, 
    a.RelatedTxID, 
    ISNULL(b1.OrderDescription, b2.OrderDescription)
from Table_A a
left outer join Table_B b1 on a.TxID = b1.TxID
left outer join Table_B b2 on a.RelatedTxID = b2.TxID