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Adding tables that will have AccountID as a foreign key

I have a table called Accounts which has a primary key called AccountID which is an integer. I would like to add some tables to the database that will have AccountID as a foreign key. One of these tables is a list of transactions per account. The Transactions table will have the following two columns:
* TransactionID int NOT NULL
* AccountID int NOT NULL (foreign key)
I am having trouble deciding on how to define the primary key for this table. Should it be:

Option 1:
I.e. each row will have a unique TransactionID.

Option 2:
CONSTRAINT (PK_Transactions) PRIMARY KEY (AccountID, TransactionID)
I.e. For each AccountID, there will be a unique transactionID. E.g. If
AccountID 99 has 2 transactions, the Transactions table will look like this
AccountID  TransactionID
---------  -------------
99         1 
99         2

Which option do you recommend? Are there any other options I should be considering? Thanks!

I would recommend a third option (which might actually be your first option).

CREATE TABLE t_transactions (
   TransactionID        INT              IDENTITY
,  AccountID            INT              NOT NULL
   CONSTRAINT XFK01t_transactions
      FOREIGN KEY (AccountID) REFERENCES t_accounts (AccountID)
-- other columns as needed

This gives you a clean PK (Primary Key), so that if you need to switch a transaction from one account to another, you don't have to worry about renumbering anything. It also preserves the relationship between the transaction and the corresponding account. I think that this gives you the best of both worlds.

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