Question

我试图避免使用循环来获得此解决方案的冲动，而是使用基于集合的操作。

我需要根据最近出现的连接到另一个表来更新表中的每条记录。我的第一个想法是加入我需要＆＃34;最近的约会＆＃34;来自和加入条件的分组，但使用GROUP BY与UPDATE无效。

这是我到目前为止所拥有的。我有一种感觉，这将通过CTE来解决......

CREATE TABLE Part (
    Area            VARCHAR(10) NOT NULL,
    SequenceNumber  INT         NOT NULL,
    DateAdded       DATETIME    NOT NULL
);
INSERT INTO Part VALUES
('A', 1, '2018-04-01'),
('A', 1, '2018-04-02'),
('A', 2, '2018-04-03'),
('A', 2, '2018-04-04'),
('B', 2, '2018-04-04'),
('B', 2, '2018-04-03'),
('B', 3, '2018-04-02'),
('B', 3, '2018-04-01');

DECLARE @Filters    TABLE
(
    Area            VARCHAR(10),
    StartingSeqNum  INT,
    StartTime       DATETIME 
);
INSERT INTO @Filters VALUES
('A', 2, NULL),
('B', 3, NULL);

-- Update each StartTime in @Filters with the most recent
-- DateAdded of the corresponding SequenceNumber in the Part table per Area.
UPDATE f
SET f.StartTime = MAX(p.DateAdded)
FROM
    @Filters f
    INNER JOIN Part p ON p.Area = f.Area
        AND p.SequenceNumber = f.StartingSeqNum
GROUP BY
    p.Area,
    p.SequenceNumber;

根据上面的示例数据，操作应填充@Filters中的以下值：

('A', 2, '2018-04-04')
('B', 3, '2018-04-02')

Answer 1

您可以使用CTE执行此操作，如下所示：

;with maxParts as
(
    select Area, SequenceNumber, MAX(dateadded) as maxDateAdded 
    from Part
    group by Area, SequenceNumber
)
UPDATE f
SET f.StartTime = p.maxDateAdded
from @Filters f
    inner join maxParts p 
        on f.Area = p.Area
        and f.StartingSeqNum = p.SequenceNumber

但是，您也可以将CTE重写为子查询：

UPDATE f
SET f.StartTime = p.maxDateAdded
from @Filters f
    inner join (
            select Area, SequenceNumber, MAX(dateadded) as maxDateAdded 
            from Part
            group by Area, SequenceNumber
        ) p 
        on f.Area = p.Area
        and f.StartingSeqNum = p.SequenceNumber

这些的查询计划应该是相同的 - 所有CTE确实允许您查看更清晰的查询以及轻松重用该SELECT的能力。

Answer 2

（a）中

UPDATE Filters
       SET Filters.StartTime = (SELECT max(Part.DateAdded)
                                FROM Part
                                WHERE Part.Area = Filters.Area
                                      AND Part.SequenceNumber = Filters.StartingSeqNum)
       FROM @Filters Filters;

（b）中

UPDATE Filters
       SET Filters.StartTime = X.StartTime
       FROM @Filters Filters
            INNER JOIN (SELECT max(Part.DateAdded) StartTime,
                               Part.Area,
                               Part.SequenceNumber
                               FROM Part
                               GROUP BY Part.Area,
                                        Part.SequenceNumber) X
                       ON X.Area = Filters.Area
                          AND X.SequenceNumber = Filters.StartingSeqNum;

两者，（a）和（b）应该做你想要的。我不确定什么更好。也许（b）可以重用子查询关系，因此整体成本较低。

通过连接到满足某些条件的另一个表的最新记录来更新表

2 个答案: