Question

这会产生语法错误：

SELECT id, name, body, DISTINCT(sequence) FROM `contents` 
WHERE (`contents`.section_id = 74) 
ORDER BY sequence ASC, revision DESC

Hokay，所以，在我的表中，我想根据序列号选择具有最新版本号的记录。

所以...我可以有这样的记录

seq | rev
0 | 2
0 | 1
0 | 0
1 | 1
1 | 0
2 | 3
2 | 1
2 | 0

我正在尝试编写一个查询来返回以下内容

   seq | rev
    0 | 2
    1 | 1
    2 | 3

注意：为了清洁问题，我遗漏了所有其他栏目笔记2：确切地说，（序列，修订）组合总是唯一的。

Answer 1

此查询选择修订与同一序列的最大修订版匹配的所有记录，并返回同一记录的其他字段。

SELECT c.id, c.name, c.body, c.sequence 
FROM contents c
WHERE 
  c.section_id = 74 AND
  IFNULL(c.revision, -1) = 
    (SELECT
      IFNULL(MAX(cx.revision), -1)
    FROM
      contents cx 
    WHERE
      cx.section_id = 74 AND /* Added this */
      cx.sequence = c.sequence)
ORDER BY sequence

Answer 2

子查询：

SELECT sequence, max(revision) as maxrev FROM contents 
WHERE section_id = 74
GROUP BY sequence

会给你一个带有（序列，修订版）对的结果，用于挑选你想要的记录（正如你所说的那样，这对值在表中是唯一的）。使用原始表的内部联接来挑选其他列：

SELECT c.id, c.name, c.body, c.sequence 
FROM contents as c INNER JOIN 
    (SELECT sequence, max(revision) as maxrev FROM contents
     WHERE section_id = 74 GROUP BY sequence) as p 
    ON (c.sequence = p.sequence AND c.revision = p.maxrev)
WHERE c.section_id = 74
ORDER BY c.sequence

根据（序列，修订版）对在整个表中是唯一的，还是仅在具有相同section_id的一组记录中，外部查询中的WHERE子句可能是多余的。

Answer 3

GROUP BY和MAX（）在你的情况下不起作用吗？

SELECT 
id,
name, 
body, 
sequence as seq,
MAX(rev) as m_rev
FROM 
`contents` 
WHERE (`contents`.section_id = 74) 
GROUP BY sequence 
ORDER BY sequence ASC, rev DESC

Answer 4

可能会使用以下内容：

SELECT id, name, body, sequence FROM contents c 
WHERE section_id = 74 AND revision in ( SELECT MAX(revision) FROM contents WHERE 
sequence=c.sequence) ORDER BY sequence ASC

MySQL：如何在列上使用distinct，但仍然选择所有列？

4 个答案: