sql query help on multiple count columns and group by
i have the following table Students:
id | status | school | name
----------------------------
0 | fail | skool1 | dan
1 | fail | skool1 | steve
2 | pass | skool2 | joe
3 | fail | skool2 | aaron
i want a result that gives me
school | fail | pass
---------------------
skool1 | 2 | 0
skool2 | 1 | 1
I have this but it's slow,
SELECT s.school, (
SE开发者_开发百科LECT COUNT( * )
FROM school
WHERE name = s.name
AND status = 'fail'
) AS fail, (
SELECT COUNT( * )
FROM school
WHERE name = s.name
AND status = 'pass'
) AS pass,
FROM Students s
GROUP BY s.school
suggestions?
Something like this should work:
SELECT
school,
SUM(CASE WHEN status = 'fail' THEN 1 ELSE 0 END) as [fail],
SUM(CASE WHEN status = 'pass' THEN 1 ELSE 0 END) as [pass]
FROM Students
GROUP BY school
ORDER BY school
EDIT
Almost forgot, but you could also write the query this way:
SELECT
school,
COUNT(CASE WHEN status = 'fail' THEN 1 END) as [fail],
COUNT(CASE WHEN status = 'pass' THEN 1 END) as [pass]
FROM Students
GROUP BY school
ORDER BY school
I'm not sure if there's any performance benefit with second query. My guess would be if there is it's probably very small. I tend to use the first query because I think it's more clear but both should work. Also, I don't have a MySql instance handy to test with, but according to @Johan the ORDER BY clauses are unnecessary.
SELECT q.school, q.fail, q.failpass-q.fail as pass
FROM
(
SELECT s.school, sum(if(status = 'fail',1,0)) as fail, count(*) as failpass
FROM students s
GROUP BY s.school
) q
This way you save one conditional sum.
In MySQL a GROUP BY
already orders the results, so a separate ORDER BY
is not needed.
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