MySQL 5.1.7有条件地联接两个表，从第二个表中按日期选择顶部记录，从第一个表中选择所有记录

Question

I have two tables:

• instructions — one unique record for each person
• events — multiple records for each person; records are of various types

I need to retrieve the compete list of instructions and, for each one, the 'appointment' event with the greatest (latest) date.

I've experimented with group by , but haven't managed to get the right results.

The instructions table:

    id    first_name    surname    telephone
   ----  ------------  ---------  -----------
    1     bob           marley     555-1234
    2     steve         pike       555-3456
    3     daniel        osborne    555-9876
    4     mark          hodge      555-6600
    5     stefan        belfant    555-8080

The events table:

    id    instruction_id    type        commment    event_date
   ----- ----------------  ------------ ---------  ------------
    1     1                 create      na          2013-11-18
    2     2                 call        na          2013-11-19
    3     2                 appointment onsite      2013-12-02
    4     3                 create      na          2013-09-17
    5     3                 appointment office      2013-09-17
    6     3                 finalize    as discuss  2013-11-19
    7     4                 create      na          2013-12-02
    8     4                 cancel      na          2013-12-02
    9     5                 create      na          2013-10-02 
    10    1                 appointment at home     2013-11-22 
    11    2                 appointment at home     2013-12-05

The desired output:

    id   first_name     surname     telephone   appointment   comment
   ---- ------------   ---------   ----------- ------------- ---------
    1    bob            marley      555-1234    2013-11-22    at home  
    2    steve          pike        555-3456    2013-12-05    at home
    3    daniel         osborne     555-9876    2013-09-17    office
    4    mark           hodge       555-6600
    5    stefan         belfant     555-6060

Answer 1

Assuming that the latest/greatest appointment-date for any given person is unique, you can write:

SELECT instructions.*,
       latest_events.event_date AS appointment,
       latest_events.comment
  FROM instructions
  LEFT
 OUTER
  JOIN ( SELECT events.instruction_id,
                MAX(events.event_date) AS latest_event_date
           FROM events
          WHERE events.type = 'appointment'
          GROUP
             BY events.instruction_id
       ) latest_event_dates
    ON instructions.id = latest_event_dates.instruction_id
  LEFT
 OUTER
  JOIN events latest_events
    ON latest_event_dates.instruction_id = latest_events.instruction_id
   AND latest_event_dates.latest_event_date = latest_events.event_date
   AND 'appointment' = latest_events.type
;

(If the latest/greatest appointment-date is not unique, then you'll need to add another step in the join where you select the MIN(events.id) or MAX(events.id) or whatnot for a given events.instruction_id and events.event_date .)

Answer 2

Try this

SELECT I.id,
       I.first_name,
       I.surname,
       I.telephone,
       E.event_date,
       E.comment
FROM instructions I
LEFT JOIN
  (SELECT *
   FROM
     (SELECT *
      FROM EVENTS
      WHERE EVENTS.type='appointment'
      ORDER BY EVENTS.event_date DESC) EVENTS
   GROUP BY EVENTS.type,
            EVENTS.instruction_id ) E ON I.id=E.instruction_id

Check Sql Fiddle

Answer 3

In the end the solution came from one of the staff working for me.... It's lightening quick and exactly what is required:

SELECT i.id, ins.type, ins.event_date FROM instructions i left join instructions i on ins.id = (select inss.id FROM events inss WHERE inss.instruction_id = i.id AND inss.type = 'Appointment' ORDER BY inss.event_date DESC LIMIT 1 );

The Group By method, no matter how I chose to do it, always returned odd results;

thank you everyone for your help on this, greatly appreciated.