Mysql 左连接一行

Question

我有三张表

Drivers

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

每个Driver都有班次

  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

并且在每个轮班期间，driver 可能会或可能不会进行多次旅行

     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

我想获得一个查询，其中我获得了最近的大部分行程 以及driver 详细信息，在开放班次中（结束时间为空）。

我一直在尝试的一切都失败了。

我理解查询应该是这样的：

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;

请帮我查询一下。预期结果类似于：

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

这应该可以满足您的需求。

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 添加并修复了列名：http://sqlfiddle.com/#!9/faeca/5

Answer 2

您可以使用变量来确定每个班次分区内的最近行程：

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

谓词 t.row_number = 1 有效地选择每个班次 最近的行程 。查询的其余部分只是简单的 JOIN 子句，将驾驶员和（打开的）班次数据收集在一起。

SQL Fiddle Demo

Mysql 左连接一行

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