Finding start time of the last sequence in mysql-CodePudding

I have following table where state 0 is for inactive device and 1 for active device.

CREATE TABLE `devices` (
  `id` int NOT NULL AUTO_INCREMENT,
  `state` int unsigned NOT NULL,
  `dt` datetime NOT NULL,
  `name` varchar(60) COLLATE utf8_unicode_ci DEFAULT NULL,
   PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO devices (state,name,dt) VALUES (1,'Alfa','2021-10-12 11:30:00');
INSERT INTO devices (state,name,dt) VALUES (0,'Alfa','2021-10-12 11:40:00');
INSERT INTO devices (state,name,dt) VALUES (1,'Alfa','2021-10-12 11:50:00');
INSERT INTO devices (state,name,dt) VALUES (1,'Alfa','2021-10-12 12:00:00');

INSERT INTO devices (state,name,dt) VALUES (1,'Beta','2021-10-12 11:30:00');
INSERT INTO devices (state,name,dt) VALUES (0,'Beta','2021-10-12 11:40:00');
INSERT INTO devices (state,name,dt) VALUES (0,'Beta','2021-10-12 11:50:00');
INSERT INTO devices (state,name,dt) VALUES (0,'Beta','2021-10-12 12:00:00');

http://sqlfiddle.com/#!9/8ea07de/3

Expected result of the query is:

Name    State   FROM                    TO
Alfa    1       '2021-10-12 11:50:00'   '2021-10-12 12:00:00'
Beta    0       '2021-10-12 11:40:00'   '2021-10-12 12:00:00'

Currently I can get only columns 'name', 'to' and 'state'. Is it possible to find query with expected result (column 'from') for MySQL 5.6?

CodePudding user response：

You can use next query:

SET @state = -1; -- initial value not equal to real values
SET @dt = '';    -- initial value not equal to real dates

SELECT 
    name,
    state,
    from_dt,
    dt
FROM (
  -- get last dt by name
  SELECT name, max(dt) dt
  FROM devices
  GROUP BY name
) last_state
JOIN (
  SELECT 
    name,
    state,
    dt,
    -- store dt eachtime when state changes
    @dt := IF(@state <> state, dt, @dt) from_dt,
    @state := state
  FROM devices
  ORDER BY name, dt ASC
) first_state using (name, dt)
;

MySQL fiddle