SQL，从两个表中选择，而不获取所有行

我有两个表：

COMMENT

和

COMMENTHISTORY

现在我需要从两个表中选择单元格，如下所示：

SELECT c.Id, c.Userid, ch.Text, ch.Timestamp
FROM COMMENT c, COMMENTHISTORY ch
WHERE ch.CommentId = c.Id
ORDER BY ch.Timestamp DESC

---

很好。唯一的问题是

COMMENTHISTORY

中的每个

注释都有几行，因此
SELECT
最终会为每个注释检索几行

我需要的是为每个注释检索一行。ch.Text和ch.Timestamp与最新相关的
COMMENTHISTORY
行匹配的行

有什么办法吗

谢谢。
这可能会帮助您：

SELECT c.Id, c.Userid, ch.Text, max(ch.Timestamp)
FROM COMMENT c, COMMENTHISTORY ch
WHERE ch.CommentId = c.Id
GROUP BY C.ID, c.userid, ch.text

SELECT c.Id, c.Userid, ch.Text, ch.Timestamp
FROM COMMENT c LEFT JOIN COMMENTHISTORY ch ON ch.CommentId = c.Id
ORDER BY ch.Timestamp DESC

使用以下命令：

SELECT c.Id, c.Userid, Max(ch.Text), max(ch.Timestamp)
FROM COMMENT c, COMMENTHISTORY ch
WHERE ch.CommentId = c.Id
GROUP BY C.ID, c.Userid
ORDER BY ch.Timestamp DESC

这将解决您的问题

SELECT c.Id, c.Userid, ch.Text, ch.Timestamp
FROM COMMENT c LEFT JOIN COMMENTHISTORY ch on C.ID = ch.comment_id
WHERE ch.CommentId = c.Id
GROUP BY c.id
ORDER BY ch.Timestamp DESC

此查询将为给定注释id的最新记录选择last
CommentHisotry
record：

WITH LastComment AS
(
    SELECT CH.CommentId, MAX(CH.Timestamp) as Date
    FROM COMMENTHISTORY CH
    GROUP BY CH.CommentId
)

SELECT c.Id, c.Userid, ch.Text, ch.Timestamp
FROM COMMENT c
INNER JOIN LastComment lc ON LC.CommentId = c.Id
INNER JOIN COMMENTHISTORY ch ON ch.CommentId = lc.CommentId and ch.TimeStamp = lc.Date
ORDER BY ch.Timestamp DESC

尝试使用内部联接，并将条件放入Where子句中。有关更多信息，请查看以下内容：-
查询：

SELECT c.Id, 
       c.Userid, 
       ch.Text, 
       ch.Timestamp
FROM COMMENT c
INNER JOIN COMMENTHISTORY ch
  ON ch.CommentId = c.Id
WHERE ch.Timestamp = (SELECT MAX(ch2.Timestamp)
                      FROM COMMENTHISTORY ch2
                      WHERE ch2.CommentId = c.Id )
ORDER BY ch.Timestamp DESC

这应该可以

;WITH CTE AS
(
    SELECT  ch.CommentId, ch.Text, ch.TimeStamp, ROW_NUMBER() OVER (PARTITION BY     ch.CommentId ORDER BY ch.TimeStamp DESC) RowNum
    FROM    COMMENTHISTORY ch
)
SELECT          c.Id, c.Userid, CTE.Text, CTE.TimeStamp
FROM            COMMENT c
    INNER JOIN  CTE ON c.Id = CTE.CommentId
WHERE           RowNum = 1

我认为内部连接会对您有所帮助。试着用另外两个SQL Server/Oracle/MySQL最大化ch.Text和ch.Timestamp并分组？请修改问题以添加您正在使用的数据库的标记
COMMENTHISTORY
has row ID column？-ANSI-92 SQL标准（20多年前！）停止了旧式的逗号分隔的表格列表样式。
GROUP BY
除了聚合的列之外，不包含所有必需的列吗？既然您使用的是时间戳的
MAX
，是否需要
ORDER BY
（ch.Text），max（ch.Timestamp）

可能不会返回同一行，因为ORDER BY子句中的“COMMENTHISTORY.Timestamp”列无效，因为它不包含在聚合函数或GROUP BY子句中。同样

max（ch.Text），max（ch.Timestamp）

可能不会返回同一行。此查询提供的结果与我自己的查询完全相同（即所有COMMENTHISTORY行，而不仅仅是最新的一行）

;WITH CTE AS
(
    SELECT  ch.CommentId, ch.Text, ch.TimeStamp, ROW_NUMBER() OVER (PARTITION BY     ch.CommentId ORDER BY ch.TimeStamp DESC) RowNum
    FROM    COMMENTHISTORY ch
)
SELECT          c.Id, c.Userid, CTE.Text, CTE.TimeStamp
FROM            COMMENT c
    INNER JOIN  CTE ON c.Id = CTE.CommentId
WHERE           RowNum = 1