Catching latest column value change in SQL - mysql

Question

How can I get the date for the latest value change in one column with one SQL query?
Possible database situation:
Date State
2012-11-25 state one
2012-11-26 state one
2012-11-27 state two
2012-11-28 state two
2012-11-29 state one
2012-11-30 state one
So result should return 2012-11-29 as latest change state. If I group by State value, I will get the date for first time I have that state in database.
The query will group the table on state and show the state and in the date field the latest date created of that state.
From the given input the output would be
Date State
2012-11-30 state one
2012-11-28 state two

Answer 1

This will get you the last state:
-- Query 1
SELECT state
FROM tableX
ORDER BY date DESC
LIMIT 1 ;
Encapsulating the above, we can use it to get the date just before the last change:
-- Query 2
SELECT t.date
FROM tableX AS t
JOIN
( SELECT state
FROM tableX
ORDER BY date DESC
LIMIT 1
) AS last
ON last.state <> t.state
ORDER BY t.date DESC
LIMIT 1 ;
And then use that to find the date (or the whole row) where the last change occurred:
-- Query 3
SELECT a.date -- can also be used: a.*
FROM tableX AS a
JOIN
( SELECT t.date
FROM tableX AS t
JOIN
( SELECT state
FROM tableX
ORDER BY date DESC
LIMIT 1
) AS last
ON last.state <> t.state
ORDER BY t.date DESC
LIMIT 1
) AS b
ON a.date > b.date
ORDER BY a.date
LIMIT 1 ;
Tested in SQL-Fiddle
And a solution that uses MySQL variables:
-- Query 4
SELECT date
FROM
( SELECT t.date
, #r := (#s <> state) AS result
, #s := state AS prev_state
FROM tableX AS t
CROSS JOIN
( SELECT #r := 0, #s := ''
) AS dummy
ORDER BY t.date ASC
) AS tmp
WHERE result = 1
ORDER BY date DESC
LIMIT 1 ;

Answer 2

I believe this is the answer:
SELECT
DISTINCT State AS State, `Date`
FROM
Table_1 t1
WHERE t1.`Date`=(SELECT MAX(`Date`) FROM Table_1 WHERE State=t1.State)
...and the test:
http://sqlfiddle.com/#!2/8b0d8/5

Answer 3

If you add another column 'changed datetime' you can fill this using an update trigger that inserts NOW(). If you query your table ordering on the changed column, it will endup first.
CREATE TRIGGER `trigger` BEFORE UPDATE ON `table`
FOR EACH ROW
BEGIN
SET ROW.changed = NOW();
END$$

Answer 4

Try this ::
Select
MAX(`Date`), state from mytable
group by state

Answer 5

If you had been using postgres, you could compare different rows in the same table using "LEAD .. OVER" I have not managed to find the same functionallity in mysql.
A bit hairy, but I think this will do:
select min(t1.date) from table_1 t1 where
(select count(distinct state) from table_1 where table_1.date>=t1.date)=1
Basically, this asks for the first time no changes in state is found for any later values. Be warned, it may be this query scales terribly for large data sets....

Answer 6

I think your best choice here are analytical functions. Try this - it should be OK performance-wise:
SELECT *
FROM test
WHERE my_date = (SELECT MAX (my_date)
FROM (SELECT MY_DATE
FROM ( SELECT MY_DATE,
STATE,
LAG (state) OVER (ORDER BY MY_DATE)
lag_val
FROM test
ORDER BY MY_DATE) a
WHERE state != lag_val))
In the inner select, the LAG function gets the previous value in the STATE column and in the outer select I mark the date of a change - those with lag value different than the current state value. And outside, I'm getting the latest date from those dates of a change... I hope that this is what you needed.

Answer 7

SELECT MAX(DATE) FROM YOUR_TABLE
Above answer doesn't seem to satisfy what OP needs.
UPDATED ANSWER WITH AFTER INSERT/UPDATE TRIGGER
DELCARE #latestState varchar;
DELCARE #latestDate date;
CREATE TRIGGER latestInsertTrigger AFTER INSERT ON myTable
FOR EACH ROW
BEGIN
IF OLD.DATE <> NEW.date THEN
SET #latestState = NEW.state
SET #latestDate = NEW.date
END IF
END
;
CREATE TRIGGER latestUpdateTrigger AFTER UPDATE ON myTable
FOR EACH ROW
BEGIN
IF OLD.DATE = NEW.date AND OLD.STATE <> NEW.STATE THEN
SET #latestState = NEW.state
SET #latestDate = NEW.date
END IF
END
;
You may use the following query to get the latest record added/updated:
SELECT DATE, STATE FROM myTable
WHERE STATE = #latestState
OR DATE = #latestDate
ORDER BY DATE DESC
;
Results:
DATE STATE
November, 30 2012 00:00:00+0000 state one
November, 28 2012 00:00:00+0000 state two
November, 27 2012 00:00:00+0000 state two
The above query results needs to be limitted to 2, 3 or n based on what you need.
Frankly it seems like you want to get max from both columns based on the data sample you have given. Assuming that your state only increases with the date. Only I wish if the state was an integer :D
Then union of two max sub queries on both columns would have solved it easily. Still a string manipulation regex can find what's the max in state column. Finally this approach needs limit x. However it still has lope hole. Anyway it took me sometime to figure your need out :$