SQL Group BY using strings in new columns

问题

I have a database of race results. Each race event has multiple classes.

events table:

event_id | event_date
---------+------------
1        | 11/5/14
2        | 11/12/14
3        | 11/19/14

results table:

result_event | name          | class | position
-------------+---------------+-------+-----------
1            | Jason Smith   | 40    | 1
1            | David Johnson | 40    | 2
1            | Randy White   | 30    | 1
1            | Billy Hansen  | 30    | 2 
2            | Wally Mann    | 40    | 1
2            | Shawn Little  | 40    | 2
2            | Eric Davis    | 30    | 1
2            | Tom Handy     | 30    | 2

I want to create a summary table that lists the Event Date and the winners of each class.

Like this:

Event Date  | Class 40 Winner | Class 30 Winner
------------+-----------------+------------------
11/5/14     | Jason Smith     | Randy White
11/12/14    | Wally Mann      | Eric Davis

What query would I need so that I can create a GROUP BY event_id and list winners in separate columns?

回答1:

You can join the events table on two queries from the results table, one for each class:

SELECT    event_data, class_40_winner, class_30_winner
FROM      events e
LEFT JOIN (SELECT result_event, name AS class_40_winner
           FROM   results 
           WHERE  class = 40 AND position = 1) c40 ON e.id = c40.result_event
LEFT JOIN (SELECT result_event, name AS class_30_winner
           FROM   results 
           WHERE  class = 30 AND position = 1) c30 ON e.id = c30.result_event

回答2:

You are querying like pivoting data, so I suggest you to use a query like this:

select event_date
    , max(case when r.class = 40 then name end) `Class 40 Winner`
    , max(case when r.class = 30 then name end) `Class 30 Winner`
from events e
left join results r on e.event_id = r.result_event and r.position = 1
group by event_date;

[SQL Fiddle Demo]

来源：https://stackoverflow.com/questions/31012881/sql-group-by-using-strings-in-new-columns

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mysql

sql

select

group-by