Creating new table which contains unique rows with conditions ; MySQL - mysql

Question

I have a table with the following columns:
SessionID - contains actions within a session (1-10 rows per SessionID let's say)
ActionName - is the name of the action
Time - time action occurred
I need to return a new table, whose columns are the same, and contains only 1 row per SessionID IF the action's name is either "a" or "b".
That is, my new table should have a 1 row per SessionID which had the action "a" or "b".
I tried:
CREATE TABLE U_SessionID (
SELECT DISTINCT(SessionID) AS SessionID FROM test1)
;
I copy the unique sessions to a new table
SELECT U_SessionID.SessionID, test1.ActionName, test1.SessionID
FROM test1
INNER JOIN U_SessionID ON (SELECT SessionID
FROM test1
WHERE U_SessionID.SessionID = test1.SessionID AND (ActionName =
"a" OR ActionName = "b")
ORDER BY Time DESC
LIMIT 1);
But this code causes MySQL workbench to crash (timeout performing query), and I have no idea if it even works.
Sample data:
Can you think of a lighter query to run this?
Maybe a better approach would be:
take all rows with action "a" or action "b":
SELECT * FROM test1 WHERE ActionName = "a" OR ActionName = "b";
drop duplicates based on the SessionID only (no matter the time order)
Ideas for that?

Answer 1

Is the time unique for a session ID?
If so:-
SELECT U_SessionID.SessionID, test1.ActionName, test1.SessionID
FROM test1
INNER JOIN
(
SELECT SessionID
MAX(Time) AS MaxTime
FROM test1
WHERE ActionName IN ("a", "b")
GROUP BY SessionID
) sub0
ON test1.SessionID = sub0.SessionID
INNER JOIN U_SessionID
ON sub0.SessionID = U_SessionID.SessionID
AND sub0.MaxTime = U_SessionID.Time
If time isn't unique, but assuming the table has a unique column called id:-
SELECT U_SessionID.SessionID, test1.ActionName, test1.SessionID
FROM test1
INNER JOIN
(
SELECT SessionID
SUBSTRING_INDEX(GROUP_CONCAT(id ORDER BY Time DESC), ',', 1) AS MaxId
FROM test1
WHERE ActionName IN ("a", "b")
GROUP BY SessionID
) sub0
ON test1.SessionID = sub0.SessionID
INNER JOIN U_SessionID
ON sub0.SessionID = U_SessionID.SessionID
AND sub0.MaxId = U_SessionID.id
EDIT
If you just want a random record for each session id then you could (ab)use the GROUP BY clause. I really do not like this idea as while it probably does work, it might well not work depending on the configuration of your MySQL database, and even if it does work someone might do an update that stops it working. Even then if it still works there is no garuntee that it will bring back a single real record, rather than a single rows whose columns are a mix of those from different rows.
So put here for completeness and to give you an option but I strongly advise against using it
SELECT U_SessionID.SessionID, test1.ActionName, test1.SessionID
FROM test1
INNER JOIN U_SessionID ON U_SessionID.SessionID = test1.SessionID
WHERE ActionName IN ("a" , "b")
GROUP BY U_SessionID.SessionID
If you want a simple solution that is more future proof then use my 2nd solution and remove the ORDER BY clause within the GROUP_CONCAT

Answer 2

You can change you query like below. Your subquery is unnecessary here.
SELECT U_SessionID.SessionID, test1.ActionName, test1.SessionID
FROM test1
INNER JOIN U_SessionID ON U_SessionID.SessionID = test1.SessionID
WHERE ActionName IN ("a" , "b")
ORDER BY `Time` DESC
LIMIT 1;