Awhile back I got some help with a specific query. Here's the link: SQL Group BY using strings in new columns
My query looks similar to this:
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
I have now entered enough data in my database (22,000 rows) that this query is taking over 6 seconds to complete. (My actual query is bigger than the above, in that it now has 4 joins in it.)
I used the "Explain" function on my query to take a look. Each of the queries from the "results" table is pulling in the 22,000 rows, so this seems to be the problem.
I have done some research and it sounds like I should be able to INDEX the relevant column on the "results" table to help speed things up. But when I did that, it actually slowed my query down to about 10 seconds.
Any suggestions for what I can do to improve this query?
AFAIK, you are pivoting your data and I think using max(case ...) ... group by
has good performance in pivoting data.
I can suggest you to use this query instead:
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;
Try this query:
SELECT
e.event_date,
r1.name as class_40_winner,
r2.name as class_30_winner
FROM
events e,
results r1,
results r2
WHERE
r1.class = 40 AND
r2.class = 30 AND
r1.position = 1 AND
r2.position = 1 AND
r1.result_event = e.id AND
r2.result_event = e.id
SELECT e.event_data
, r.class
, r.name winner
FROM events e
JOIN results r
ON r.result_event = e.id
WHERE class IN (30,40)
AND position = 1
The rest of this problem is a simple display issue, best resolved in application code.
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