Question

I have table of user actions, each having a user associated, a type, and a timestamp. Here’s a simplified example:

TABLE USER_ACTIONS
------------------------
USER | TYPE  | TIMESTAMP
------------------------
a    | OPEN  | 0
b    | OPEN  | 1
a    | CLOSE | 2
a    | OPEN  | 3
b    | CLOSE | 4
a    | CLOSE | 4
a    | OPEN  | 5  <-- "orphaned" OPEN, with no corresponding CLOSE. Should be ignored.
c    | OPEN  | 3
c    | CLOSE | 5
a    | OPEN  | 6
a    | CLOSE | 8

I’d like to get a list of transaction times out of this. Each CLOSE should match the previous OPEN, for a particular user.

The results I’d like will look something like this:

USER | TRANSACTION_TIME
-----------------------
a    | 2
b    | 3
a    | 1
c    | 2
a    | 2

I don’t care about the ordering.

I know that this is possible to do programmatically, but is it possible to do with some clever SQL?

UPDATE:

To do this programmatically, the general idea would be to…

1. Select all of the “CLOSE” actions, ordering by TIMESTAMP descending.
2. For each of those in that list, try to find a previous “OPEN” action made by the same user. Limit the TIMESTAMP to be before the “CLOSE” action TIMESTAMP, sort the results by TIMESTAMP DESC, and limit them to 1.
3. For that pair, calculate the time difference, and ouput the result.

Here’s some pseudocode, but really I’d like SQL that does this cleverly:

for each CLOSE_ACTION IN ("SELECT USER, TYPE, TIMESTAMP FROM USER_ACTIONS WHERE TYPE='CLOSE' ORDER BY TIMESTAMP DESC;") {
    OPEN_ACTION = "SELECT USER, TYPE, TIMESTAMP FROM USER_ACTIONS
                   WHERE TYPE='OPEN'
                   AND USER='<CLOSE_ACTION.USER>'
                   AND TIMESTAMP='<CLOSE_ACTION.TIMESTAMP>'
                   ORDER BY TIMESTAMP DESC
                   LIMIT 1";
    if OPEN_ACTION != empty/null then {
        print CLOSE_ACTION.USER, CLOSE_ACTION.TIMESTAMP - OPEN_ACTION.TIMESTAMP;
    }
}

Answer 1

This takes each CLOSE event and matches it to the preceding event, if and only if the preceding event is an OPEN.

SELECT
  OPEN.user,
  OPEN.transaction_time
  CLOSE.transaction_time
FROM
  user_actions as CLOSE
INNER JOIN
  user_actions as OPEN
    ON  OPEN.user = CLOSE.user
    AND OPEN.transaction_time = (SELECT MAX(transaction_time) FROM user_action
                                 WHERE user = CLOSE.user
                                 AND transaction_time < CLOSE.transaction_time
                                 AND type='OPEN')
WHERE
    CLOSE.type = 'CLOSE'