SQL学习——选择分组中的最大值
数据如下：
第一列为id，第二列是标签标号，第三列是时间，第四列是房间id
|------------ID------------||-------------------------rfid-------------------------||---------------------time-------------------||-----------------------------room_id----------------------------||
0 100000000000000020120001 20130716 09:24:07 8a8191913d826b19013daae03c5f0f69
1 100000000000000020120002 20130712 09:24:07 8a8191913d826b19013daae03c5f0f60
2 100000000000000020120001 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
3 100000000000000020120003 20130712 09:24:07 8a8191913d826b19013daae03c5f0f60
4 100000000000000020120002 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
5 100000000000000020120003 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
|------------ID------------||-------------------------rfid-------------------------||---------------------time-------------------||-----------------------------room_id----------------------------||
现在需要从中选出每个标签号时间最新的房间id。
刚开始想到用Group By来查，
SELECT max(time),rfid,id, room_id FROM TABLE_TEST GROUP BY rfid ORDER BY ID;
比较纠结的是，查询结果的room_id和time对不上号。
网上找了下，有用内联查询的
select A.id,A.time,A.rfid, A.room_id from TABLE_TEST A inner join (select rfid, max(time) as time fromTABLE_TEST group by rfid) B on A.rfid=B.rfid AND A.time=B.time;
查询结果一看，对了！！！
但是这个查询还有一个过滤不了，就是如果有两条最新的记录，比如说
|------------ID------------||-------------------------rfid-------------------------||---------------------time-------------------||-----------------------------room_id----------------------------||
0 100000000000000020120001 20130716 09:24:07 8a8191913d826b19013daae03c5f0f69
1 100000000000000020120002 20130712 09:24:07 8a8191913d826b19013daae03c5f0f60
2 100000000000000020120001 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
3 100000000000000020120003 20130712 09:24:07 8a8191913d826b19013daae03c5f0f60
4 100000000000000020120002 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
5 100000000000000020120003 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
6 100000000000000020120003 20130714 09:24:07 8a8191913d826b19013daae03c5f0f69
|------------ID------------||-------------------------rfid-------------------------||---------------------time-------------------||-----------------------------room_id----------------------------||
这个好解决，加了个group by将就着解决问题。
select A.id,A.time,A.rfid, A.fangjid from TABLE_TEST A inner join (select id,rfid, max(time) as time from TABLE_TEST group by rfid) B on A.rfid=B.rfid AND A.time=B.time group by a.rfid;
拆开来看的话，其实也可以用两个select 组合查询
删除冗余记录：
SELECT count(*) FROM WZ_PAND;
DELETE FROM WZ_PAND WHERE id NOT IN
(
SELECT
A.id
FROM
WZ_PAND A
INNER JOIN (
SELECT
max(time) AS time,
rfid
FROM
WZ_PAND
GROUP BY
rfid
) B ON A.rfid = B.rfid
AND A.time=B.time);
SELECT count(*) FROM WZ_PAND;
SELECT * FROM WZ_PAND;
记录下SQLite批量更新的学习成果。
更新盘点表中的信息到物资表中
UPDATE WZ_WUZ
SET fangjid = (
SELECT
fangjid
FROM
WZ_PAND
WHERE
WZ_PAND.rfid = WZ_WUZ.rfid
LIMIT 1
),
state = '02',
operType = (
CASE operType
WHEN '' THEN
'02'
WHEN NULL THEN
'02'
ELSE
operType
END
)
WHERE
EXISTS (
SELECT
*
FROM
WZ_PAND
WHERE
WZ_PAND.rfid = WZ_WUZ.rfid
AND WZ_PAND.fangjid <> WZ_WUZ.fangjid
);
这个语句在SQLite下可用，加上fangjid不等于的判断，将六千条数据80秒的时间，减少到3.062s，减少了无效更新的次数。
还有个语句SQLite用不了，我也没条件试了，记下来先
UPDATE WZ_WUZ
SET (fangjid, state, operType) = (
SELECT
t2.fangjid,
'02',
CASE t3.operType
WHEN '' THEN
'02'
WHEN NULL THEN
'02'
ELSE
t3.operType
END
FROM
WZ_PAND t2
INNER JOIN WZ_WUZ t3 ON t3.rfid = t2.rfid
)