Ich habe zwei Tische Service 和 Status。服务表只保存一个name和一个id
| id | name | |----|-------| | 1 | Test1 | | 2 | Test2 |
Es gibt auch eine Statustabelle wie diese
| id | status | service_id | timestamp | |----|--------|------------|---------------------------| | 1 | OK | 1 | October, 15 2015 09:03:07 | | 2 | OK | 1 | October, 15 2015 09:08:07 | | 3 | OK | 2 | October, 15 2015 10:05:23 | | 4 | OK | 2 | October, 15 2015 10:15:23 |
Ich möchte solche Daten erhalten
| id | name | status | timestamp | |----|-------|--------|---------------------------| | 1 | Test1 | OK | October, 15 2015 09:08:07 | | 2 | Test2 | OK | October, 15 2015 10:15:23 |
Aktuellster Stand mit Servicedaten. Ich habe diese Aussage ausprobiert
SELECT ser.id, ser.name, a.status, a.timestamp
from Service ser
inner join (select * from status
order by Status.timestamp
DESC limit 1) as a
on a.service_id = ser.id
Aber alles was ich bekam war
| id | name | status | timestamp | |----|-------|--------|---------------------------| | 2 | Test2 | OK | October, 15 2015 10:15:23 |
Wie ändere ich die Aussage, um das zu bekommen, was ich will?
Zum Testen von SQL Fiddle
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对于每项服务，仅当不存在后续服务时才使用
NOT EXISTS返回状态：select ser.id, ser.name, st.status, st.timestamp from service ser left join status st1 on ser.id = st1.service_id where not exists (select 1 from status st2 where st2.service_id = st1.service_id and st2.timestamp > st1.timestamp)可以选择执行
LEFT JOIN来返回没有任何状态的服务。如果不需要，请切换到JOIN。你可以这样做：
SELECT ser.id, ser.name, s.status, s.timestamp FROM Service ser INNER JOIN status as s ON s.service_id = ser.id INNER JOIN ( SELECT service_id, MAX(timestamp) AS MaxDate FROM status GROUP BY service_id ) AS a ON a.service_id = s.service_id AND a.MaxDate = s.timestamp;与子查询的连接：
将消除除最新日期之外的所有状态。