Q

How do I retrieve second transaction for each customer?

An Oracle user asks how to write a query to retrieve the second transaction for each customer.

In a customer transaction table, I need to pick the second transaction of every customer. For instance,

Transactions
TRXID  CustID  Date       Amount
15      100    10Dec07    1000$
33      100    11Dec07    2000$ 
44      100    12Dec07    3000$ 
67      100    14Dec07    5000$

Could you please tell me how to write a query to retrieve the second TRXID (which is 33) for the customer ID 100?

The description of your problem didn't actually say so, but the sequence of transactions is determined by your Date column. You'd be surprised how many people do not realize that there is no inherent sequence in relational database tables. Sequence can only be determined in two ways: using ORDER BY, and using comparison operators. In both cases, it is the values within a column that determine sequence.

Thus the sequence of rows in your transaction table is determined by the values in the Date column, such that the first transaction is the one with the lowest date, the second transaction is the one with the second lowest date, and so on. The values of the Date column must be taken in the context of the same customer.

Here's the query:

select TRXID
     , CustID  
     , Date  
     , Amount
  from Transactions as T
 where CustID = 100
   and Date =
       ( select min(Date)
           from Transactions
          where CustID = T.CustID
            and Date >

                ( select min(Date)
                    from Transactions
                   where CustID = T.CustID ) )

In words, the transaction you want is the one where the date is the lowest date for that customer, that isn't the lowest date for that customer. The lowest date is the first, and the lowest date that isn't the first is the second.

Remove CustID = 100 and it will return the second transaction for each customer.

This was first published in April 2008

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