问题
We have the following, quite complex (at least for us) query. Since, as far as we know, there's no such thing as INTERSECT on MySQL, we are wondering how can we fix this:
(
SELECT GROUP_CONCAT(APA_T.district ORDER BY APA_T.district), t.name
FROM tbl_activity AS t
INNER JOIN tbl_activity_package AS ap ON t.id = ap.id_activity
INNER JOIN (
SELECT DISTINCT apa.district AS district, (
SELECT s1.id_activity_package
FROM tbl_activity_package_address s1
WHERE apa.district = s1.district
ORDER BY s1.id DESC
LIMIT 1
) AS idActivityPackage
FROM
tbl_activity_package_address apa
ORDER BY apa.district
) AS APA_T
ON ap.id = APA_T.idActivityPackage
GROUP BY t.name
ORDER BY APA_T.district
)
UNION DISTINCT
(
SELECT GROUP_CONCAT(DISTINCT apa2.district ORDER BY apa2.district), t2.name
FROM tbl_activity AS t2
INNER JOIN tbl_activity_package AS ap2 ON t2.id = ap2.id_activity
INNER JOIN tbl_activity_package_address AS apa2 ON ap2.id = apa2.id_activity_package
GROUP BY t2.name
ORDER BY apa2.district
)
#LIMIT 6, 6
Here are the results:
GROUP_CONCAT(APA_T.DISTRICT ORDER BY APA_T.DISTRICT) NAME
Beja,Faro,Setubal activity-1
Evora activity-2
Sintra activity-4
Braga,Sines activity-5
Santarem activity-6
Guarda,Matosinhos,Sagres activity-7
Lisboa,Montemor,Porto,Rio de Janeiro activity-8
Beja,Evora,Faro,Setubal activity-1
Faro activity-3
Here are the results as we wish they were:
GROUP_CONCAT(APA_T.DISTRICT ORDER BY APA_T.DISTRICT) NAME
Beja,Faro,Setubal activity-1
Evora activity-2
Sintra activity-4
Braga,Sines activity-5
Santarem activity-6
Guarda,Matosinhos,Sagres activity-7
Lisboa,Montemor,Porto,Rio de Janeiro activity-8
Faro activity-3
ISSUE
This line should NOT appear. No activity should appear twice.
Beja,Evora,Faro,Setubal activity-1
We understand that the UNION DISTINCT doesn't remove it, because indeed:
Beja, Faro, Setubal IS DIFFERENT THAN Beja,Evora,Faro,Setubal
HOWEVER, we wish NOT to have Evora to appear on the first result. So, it is OK as it is, the first query on the UNION does it's job as it should.
Still, that second activity-1 that appears, should be removed.
Any advice on how to solve this?
THE BIG PICTURE As you can see, this is quite a huge select that will, perhaps, get worst and slow by time. We wish to have a INFINITE SCROLL of Activities, and the first results of that Infinite Scroll, should be from Activities happening on different districts. Why? Why can't we do it "order by date" or something, you may ask.
Because if a database back-end user do insert the last 20 records, all from one single district, we will have on the infinite scroll first list results, only activities from that district APPEARING that we don't have more than that district.
So, the point is to LIST ALL the results on a certain (complex) ORDER. :) Any other, perhaps better way, would be great.
http://sqlfiddle.com/#!2/37dd94/51
回答1:
Does the below (SQL Fiddle) produce the results you are looking for. I wrapped the union so I could then sort on the name field. If you don't want it that way then you can remove it or sort on the DistCon field instead.
SELECT * FROM
(
SELECT GROUP_CONCAT(APA_T.district) AS DistCon, t.name
FROM tbl_activity AS t
JOIN tbl_activity_package AS ap ON t.id = ap.id_activity
JOIN
(
SELECT DISTINCT apa.district AS district,
(
SELECT s1.id_activity_package
FROM tbl_activity_package_address s1
WHERE apa.district = s1.district
ORDER BY s1.id DESC
LIMIT 1
) AS idActivityPackage
FROM
tbl_activity_package_address apa
ORDER BY apa.district
) AS APA_T
ON ap.id = APA_T.idActivityPackage
GROUP BY t.name
UNION
SELECT GROUP_CONCAT(apa.district), t.name
FROM tbl_activity AS t
JOIN tbl_activity_package AS ap ON t.id = ap.id_activity
JOIN tbl_activity_package_address AS apa ON ap.id = apa.id_activity_package
WHERE t.name NOT IN
(
SELECT DISTINCT t.name
FROM tbl_activity AS t
JOIN tbl_activity_package AS ap ON t.id = ap.id_activity
JOIN
(
SELECT DISTINCT apa.district AS district,
(
SELECT s1.id_activity_package
FROM tbl_activity_package_address s1
WHERE apa.district = s1.district
ORDER BY s1.id DESC
LIMIT 1
) AS idActivityPackage
FROM
tbl_activity_package_address apa
) AS APA_T
ON ap.id = APA_T.idActivityPackage
)
GROUP BY t.name
) AS Mm
ORDER BY Mm.name
回答2:
This query provides a slightly different result from that specified above because it employs slightly different rules.
Basically, it says "Give me at least one district for every activity. Where multiple districts offer the same activity, exclude any that are sole providers of another activity."
SELECT x.activity
, GROUP_CONCAT(DISTINCT x.district) districts
FROM
( SELECT a.name activity
, apa.district
FROM tbl_activity a
JOIN tbl_activity_package ap
ON ap.id_activity = a.id
JOIN tbl_activity_package_address apa
ON apa.id_activity_package = ap.id
) x
LEFT
JOIN
( SELECT a.name activity
, apa.district
FROM tbl_activity a
JOIN tbl_activity_package ap
ON ap.id_activity = a.id
JOIN tbl_activity_package_address apa
ON apa.id_activity_package = ap.id
GROUP
BY activity
HAVING COUNT(*) = 1
) y
ON y.district = x.district
AND y.activity <> x.activity
WHERE y.activity IS NULL
GROUP
BY activity;
+------------+--------------------------------------+
| activity | districts |
+------------+--------------------------------------+
| activity-1 | Beja,Setubal |
| activity-2 | Evora |
| activity-3 | Faro |
| activity-4 | Sintra |
| activity-5 | Braga,Sines |
| activity-6 | Santarem |
| activity-7 | Guarda,Sagres,Matosinhos |
| activity-8 | Lisboa,Porto,Rio de Janeiro,Montemor |
+------------+--------------------------------------+
来源：https://stackoverflow.com/questions/23550816/query-returning-one-extra-record-any-advice-on-how-to-remove-it-from-the-quer