SQL 分组依据和最大值

Question

我的表格中有以下数据:

id  name    alarmId  alarmUnit  alarmLevel

1   test    voltage  psu        warning
2   test    voltage  psu        ceasing
3   test    voltage  psu        warning
4   test    temp     rcc        warning
5   test    temp     rcc        ceasing

我只想显示有关每个列组(alarmId、alarmUnit)的最新信息，因此结果应如下所示:

3   test    voltage  psu        warning
5   test    temp     rcc        ceasing

到目前为止我已经尝试过:

SELECT MAX(id) as id,name,alarmId,alarmUnit,alarmLevel GROUP BY AlarmId,alarmUnit;

选定的 ID 似乎没问题，但选定的行与它们不对应。你能帮我吗？

Answer 1

在 Oracle、SQL Server 2005+ 和 PostgreSQL 8.4 中:

SELECT  *
FROM    (
        SELECT  *, ROW_NUMBER() OVER (PARTITION BY alarmId, alarmUnit ORDER BY id DESC) AS rn
        FROM    mytable
        ) q
WHERE   rn = 1

在MySQL中:

SELECT  mi.*
FROM    (
        SELECT  alarmId, alarmUnit, MAX(id) AS mid
        FROM    mytable
        GROUP BY
                alarmId, alarmUnit
        ) mo
JOIN    mytable mi
ON      mi.id = mo.mid

在 PostgreSQL 8.3 及以下版本中:

SELECT  DISTINCT ON (alarmId, alarmUnit) *
FROM    mytable
ORDER BY
        alarmId, alarmUnit, id DESC