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[英]Limit MAX() result to one row based on highest value in a particular field
[英]Limit query result based on field value
我有一個具有休閑結構的表account
:
| agg_type | agg_id | sequence | payload | is_snapshot | timestamp |
| "account" | "agg_1" | 1 | "..." | false | ... |
| "account" | "agg_1" | 2 | "..." | true | ... |
| "account" | "agg_1" | 3 | "..." | false | ... |
| "account" | "agg_1" | 4 | "..." | false | ... |
| "account" | "agg_1" | 5 | "..." | false | ... |
| "account" | "agg_1" | 6 | "..." | false | ... |
| "account" | "agg_1" | 7 | "..." | true | ... |
| "account" | "agg_1" | 8 | "..." | false | ... |
我需要編寫一個查詢,該查詢將從特定聚合的最新快照開始從該表中檢索所有行。 例如,在此表的情況下,查詢將返回最后兩行(序列 7 和 8)。
我認為查詢將 go 類似於
SELECT * FROM account
WHERE
agg_type='account'
AND agg_id='agg_1'
ORDER BY sequence ASC
LIMIT (???);
這是我不太確定如何實施的(???)
部分。
觀察:
簡單地說,我們可以只檢索序列大於或等於快照的最高序列 id 的所有帳戶
SELECT * FROM account a
WHERE
a.agg_type='account'
AND a.agg_id='agg_1'
AND a.sequence >=
(SELECT MAX(sequence) FROM account b WHERE a.agg_type = b.agg_type AND a.agg_id = b. agg_id AND b.is_snapshot = true)
如果您想全部完成,則將其編寫為聯接可能會更清楚:
SELECT a.*
FROM
account a
INNER JOIN
(
SELECT
agg_type,
agg_id,
MAX(sequence) as maxseq
FROM account b
GROUP BY agg_type, add_id
) maxes
ON
a.agg_type = maxes.agg_type and
maxes.agg_id = a.max_id and
a.sequence >= maxes.maxseq
這並不是說我們不能用任何一種形式完成任何一項任務(並且內部 postgres 可能無論如何都會執行它們),但我一直認為使用連接作為“這里有 10000 行,我想要只有符合這 1000 行規定的標准的 2000 行”最清楚地被認為是連接在一起的數據塊
WITH a AS ( SELECT *,row_number() over(partition BY a.agg_type,a.agg_id ORDER BY a."SEQUENCE" DESC) rnk FROM account a ) SELECT * FROM a WHERE a.rnk <= 2;
一個 window function 可以為所有(agg_type, agg_id)
組合僅使用一種排序:
with mark as (
select *,
bool_or(is_snapshot) over w as trail_true
from account
window w as (partition by agg_type, agg_id
order by sequence
rows between 1 following
and unbounded following)
)
select *
from mark
where not coalesce(trail_true, false)
order by agg_type, agg_id, sequence
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