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[英]Selecting max value while group by two columns with order on another column
[英]Find the max value of a column, then group by another column in same table
我有一個與此處解決的問題類似的問題,但是當我嘗試該解決方案時,我認為它失敗了,因為我將事情設置的不同。
我有一個帶有...的文檔表(不幸的是,由於它是舊系統,因此無法編輯該表)
+-------+--------+----------+--------+
| Docid | title | revision | linkid |
+-------+--------+----------+--------+
| 1 | docone | 1 | 1 |
| 2 | doctwo | 1 | 2 |
| 3 | docone | 2 | 1 |
|4 | docone | 3 | 1 |
+-------+--------+----------+--------+
在列出所有文檔的頁面上,我只想列出每個文檔的最新修訂版。 例如,Doc1在修訂版3上,因此我想要一個,而不是其他2。Doc2僅在修訂版1上,所以請顯示一個。
基於另一篇文章中的問題,我將查詢編寫如下:
$query_docs = "
SELECT `document`.*, doctype.*
FROM `document`
INNER JOIN doctype
ON `document`.iddoctypes = doctype.iddoctypes
WHERE `document`.revision = (
SELECT MAX(`document`.revision) AS revision
FROM `document`
)
GROUP BY `document`.linkid
ORDER BY `document`.doccreation DESC";
我不得不鏈接到另一個表來獲取文檔類型(只是為了使查詢更困難)。
只是一個瘋狂的猜測:我認為您必須在(select max(...) ...)
子查詢中group by title
添加group by title
。
因此,完整的聲明將是這樣的:
$query_docs = "
SELECT `document`.*, doctype.*
FROM `document`
INNER JOIN doctype
ON `document`.iddoctypes = doctype.iddoctypes
WHERE `document`.revision = (
SELECT MAX(`document`.revision) AS revision
FROM `document`
--GROUP BY `document`.title
GROUP BY `document`.linkid
)
GROUP BY `document`.linkid
ORDER BY `document`.doccreation DESC";
試試這個,我做了幾個小改動
SELECT document.*, doctype.*
FROM document
INNER JOIN doctype
ON document.iddoctypes = doctype.iddoctypes
WHERE document.revision = (
SELECT MAX(d1.revision)
FROM document d1
WHERE document.linkid = d1.linkid
)
ORDER BY document.doccreation DESC
我想念什么嗎? 這似乎很簡單...
SELECT x.*
FROM my_table x
JOIN (SELECT title,MAX(revision) max_revision FROM my_table GROUP BY title) y
ON y.title = x.title
AND y.max_revision = x.revision;
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