Mysql左连接一行

Question

I have three tables

Drivers

  driver_id | driver_name |driver_number
  ----------------------------------------
    1       | Driver 1   | 99999
    2       | Driver 2   | 88888

Each Driver has shifts

  shift_id | start_time            | end_time            | driver_id 
   -----------------------------------------------------------------
    4        |2015-04-02 10:09:00    |(NULL)               | 1
    3        |2015-04-02 09:19:00    |(NULL)               | 2
    2        |2015-04-02 11:09:00    |2015-04-02 19:09:00  | 1
    1        |2015-04-02 10:09:00    |2015-04-02 20:09:00  | 2

And during each shift, driver may or may NOT do multiple trips

     trip_id | start_time            | end_time            | shift_id 
       -----------------------------------------------------------------
        12       |2015-04-02 10:09:00    |(NULL)               | 4
        11       |2015-04-02 09:19:00    |(NULL)               | 4
        10       |2015-04-02 11:09:00    |2015-04-02 19:09:00  | 3
        9        |2015-04-02 10:09:00    |2015-04-02 20:09:00  | 2
        8        |2015-04-02 10:09:00    |(NULL)               | 4
        7        |2015-04-02 09:19:00    |(NULL)               | 4
        6        |2015-04-02 11:09:00    |2015-04-02 19:09:00  | 3
        5        |2015-04-02 10:09:00    |2015-04-02 20:09:00  | 4
        4        |2015-04-02 10:09:00    |(NULL)               | 4
        3        |2015-04-02 09:19:00    |(NULL)               | 4
        2        |2015-04-02 11:09:00    |2015-04-02 19:09:00  | 2
        1        |2015-04-02 10:09:00    |2015-04-02 20:09:00  | 1

I want to get a query where I get the recent most trip along with driver details, in the open shifts (end time null).

All that I've been trying has failed.

I understand the query should be something like this:

select * from
drivers 
inner join shifts on shifts.driver_id = drivers.driver_id
left join (some inner query on trips table) as trip 
on shifts.shift_id = trip.trip_id
where shifts.end_time is null;

Please help me with a query. The expected result is something like:

driver_id | driver_name | shift_id | shift_start_time    | recent_trip_id | recent_trip_start_time
1         | Driver 1    | 4        | 2015-05-02 10:09:00 | 12             | 2015-04-02 10:09:00
2         | Driver 2    | 3        | 2015-05-02 11:09:00 | 10             |  2015-04-02 11:09:00

Answer 1

This ought to do what you're looking for.

SELECT s.*, d.*, t.*
FROM shifts AS s 
INNER JOIN drivers AS d
    ON s.driver_id = d.driver_id
INNER JOIN (
    SELECT MAX(t.trip_id) AS trip_id
    FROM trips AS t
    INNER JOIN shifts AS s
        ON t.shift_id = s.shift_id
        AND s.end_time IS NULL
    GROUP BY s.shift_id) AS mt
INNER JOIN trips AS t
    ON s.shift_id = t.shift_id
    AND t.trip_id = mt.trip_id;

SQL Fiddle added and column names fixed: http://sqlfiddle.com/#!9/faeca/5

Answer 2

You can use variables to determine the most recent trip within each shift partition:

SELECT d.driver_id, d.driver_name, s.shift_id, s.start_time AS shift_start_time, 
       t.trip_id AS recent_trip_id, t.start_time AS recent_trip_start_time
FROM drivers AS d
INNER JOIN shifts AS s ON s.driver_id = d.driver_id
INNER JOIN (
    SELECT trip_id, start_time, 
           @row_number:= CASE WHEN @sid = shift_id THEN @row_number+1
                              ELSE 1
                         END AS row_number,
           @sid:=shift_id AS shift_id  
    FROM trips, (SELECT @sid:=0,@row_number:=0) as vars
    ORDER BY shift_id, trip_id DESC ) t ON t.shift_id = s.shift_id AND t.row_number = 1
WHERE s.end_time IS NULL

Predicate t.row_number = 1 effectively selects the most recent trip per shift . The rest of the query are just simple JOIN clauses that gather together driver and (open) shift data.

SQL Fiddle Demo