Question

I have a table that contains computer login and logoff events. Each row is a separate event with a timestamp, machine name, login or logoff event code and other details. I need to create a SQL procedure that goes through this table and locates corresponding login and logoff event and insert new rows into another table that contain the machine name, login time, logout time and duration time.

So, should I use a cursor to do this or is there a better way to go about this? The database is pretty huge so efficiency is certainly a concern. Any suggested pseudo code would be great as well.

[edit : pulled from comment]

Source table:

History (
      mc_id
    , hs_opcode
    , hs_time
)

Existing data interpretation:

Login_Event  = unique mc_id, hs_opcode = 1, and hs_time is the timestamp
Logout_Event = unique mc_id, hs_opcode = 2, and hs_time is the timestamp

Answer 1

First, your query will be simpler (and faster) if you can order the data in such a way that you don’t need a complex subquery to pair up the rows. Since MySQL doesn’t support CTE to do this on-the-fly, you’ll need to create a temporary table:

CREATE TABLE history_ordered (
  seq INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  hs_id INT,
  mc_id VARCHAR(255),
  mc_loggedinuser VARCHAR(255),
  hs_time DATETIME,
  hs_opcode INT
);

Then, pull and sort from your original table into the new table:

INSERT INTO history_ordered (
  hs_id, mc_id, mc_loggedinuser,
  hs_time, hs_opcode)
SELECT
  hs_id, mc_id, mc_loggedinuser,
  hs_time, hs_opcode
FROM history ORDER BY mc_id, hs_time;

You can now use this query to correlate the data:

SELECT li.mc_id,
       li.mc_loggedinuser,
       li.hs_time as login_time,
       lo.hs_time as logout_time
FROM   history_ordered AS li
JOIN   history_ordered AS lo
  ON   lo.seq = li.seq + 1
   AND li.hs_opcode = 1;

For future inserts, you can use a trigger like below to keep your duration table updated automatically:

DELIMITER $$
CREATE TRIGGER `match_login` AFTER INSERT ON `history`
FOR EACH ROW
BEGIN
 IF NEW.hs_opcode = 2 THEN
  DECLARE _user VARCHAR(255);
  DECLARE _login DATETIME;
  SELECT mc_loggedinuser, hs_time FROM history
  WHERE hs_time = (
   SELECT MAX(hs_time) FROM history
   WHERE hs_opcode = 1
   AND mc_id = NEW.mc_id
  ) INTO _user, _login;
  INSERT INTO login_duration
  SET machine = NEW.mc_id,
  logout = NEW.hs_time,
  user = _user,
  login = _login;
 END IF;
END$$
DELIMITER ;