为Oracle 11g重写MySQL SQL

Question

Here is the original MySQL statement:

select transactions.username,
       count(transactions.username) as total_transactions,
       last_device,
       last_transaction
  from transactions
 inner join (select username,
                    terminal     as last_device,
                    request_date as last_transaction
               from transactions t1
              where request_date =
                    (select max(request_date)
                       from transactions t2
                      where t1.username = t2.username)) as transactions_per_user
    on transactions_per_user.username = transactions.username
 group by username;

Oracle didn't like this statement, so after a lot of trial and error I rewrote it to this:

 select transactions_per_user.username, 
        count(transactions_per_user.username) as total_transactions, 
        MAX(transactions_per_user.last_transaction) as last_transaction, 
        MAX(transactions_per_user.last_device) as last_device
 from 
 transactions 
 inner join 
 (
    select username, terminal as last_device, request_date as last_transaction 
    from transactions t1 
    where request_date = (select max(request_date) 
    from transactions t2 where t1.username=t2.username)
) 
transactions_per_user on transactions_per_user.username=transactions.username 
group by transactions_per_user.username;

It seems to the right thing but I am concerned about using MAX there. I can't quite understand why it works. Can anyone verify it is ok if not help me write a statement that will work in oracle keeping close to the mysql, even better if the sql will run in both oracle and mysql.

Basically I am trying to generate a table grouped by users that lists their last transaction date and also the last device used and also a count of transactions they have made.

Answer 1

I used PL/SQL beautifier to format statements with the same format, and TortuiseSVN dif to compare them.

first query:

SELECT transactions.username,
       COUNT(transactions.username) AS total_transactions,
       last_device,
       last_transaction
  FROM transactions
 INNER JOIN (SELECT username,
                    terminal     AS last_device,
                    request_date AS last_transaction
               FROM transactions t1
              WHERE request_date =
                    (SELECT MAX(request_date)
                       FROM transactions t2
                      WHERE t1.username = t2.username)) AS transactions_per_user
    ON transactions_per_user.username = transactions.username
 GROUP BY username

second query:

SELECT transactions_per_user.username,
       COUNT(transactions_per_user.username) AS total_transactions,
       MAX(transactions_per_user.last_transaction) AS last_transaction,
       MAX(transactions_per_user.last_device) AS last_device
  FROM transactions
 INNER JOIN (SELECT username,
                    terminal     AS last_device,
                    request_date AS last_transaction
               FROM transactions t1
              WHERE request_date =
                    (SELECT MAX(request_date)
                       FROM transactions t2
                      WHERE t1.username = t2.username)) transactions_per_user
    ON transactions_per_user.username = transactions.username
 GROUP BY transactions_per_user.username;

Here are changes:

1. transactions was changed to transactions_per_user in row 1 and 2 this is fine because you inner join these data sets on transactions_per_user.username = transactions.username (but this change is not necessary)
2. last_device, last_transaction was changed with
   MAX(transactions_per_user.last_transaction) AS last_transaction,
   MAX(transactions_per_user.last_device) AS last_device
   In Oracle you can't use columns without grouping functions if they are not in group list (even if they are equal for all rows in group).
   Oracle check query before fetching data, so it doesn't know what data it will work with. (I'm not sure how this works in MySQL)
   As I understand, these columns are equal within group, so min and max is fine here.
3. AS was deleted in "AS transactions_per_user" - this is not necessary (both syntax works fine)
4. "GROUP BY username" was changed with "GROUP BY transactions_per_user.username"
   If you join with "ON" clause you need to explicitly specify table name or alias while mentioning these columns if they have equal names (otherwise Oracle doesn't know what column do you mean)
   Even if you have inner join you should use explicit alias.
   If you join with "USING" clause, then oracle merge these 2 columns into 1, and you can't use table alias for it.
   You may choose either syntax you like.