sql - SQL Group by & Max

Question

我在表中有以下数据：

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 似乎很好，但选定的行与它们不对应。你可以帮帮我吗？

Answer 1

在Oracle,SQL Server 2005+和PostgreSQL 8.4:

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

在MySQL：

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：

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

Answer 2

如果你想获得最大值的行，你可能需要一个子查询。就像是：

SELECT *
FROM YourTable
WHERE id IN (
    SELECT MAX(id) FROM YourTable GROUP BY alarmId, alarmUnit
    )

Answer 3

尝试：

SELECT * FROM table WHERE id IN
  (SELECT MAX(id) FROM table GROUP BY alarmId, alarmUnit)

Answer 4

select id, name, alarmID, alarmUnit, alarmLevel
from (select max(id) as id
from table
group by alarmID, alarmUnit) maxID
inner join table
on table.id = maxID.id

Answer 5

也许尝试以下方法：

SELECT id,name,alarmId,alarmUnit,alarmLevel
FROM table
WHERE id IN (SELECT Max(id) FROM table GROUP BY alarmId, alarmUnit)

您可能必须在子查询选择中包含 alarmId 和 alarmUnit。