基于同一张表添加和/或更新

Question

我有一个看起来像这样的表：

+--------+------+------------+
| ITEMNO | UNIT | CONVERSION |
+--------+------+------------+
| 123    | CTN  | 50         |
+--------+------+------------+
| 456    | CTN  | 300        |
+--------+------+------------+
| 789    | CTN  | 200        |
+--------+------+------------+

我想要的是有一个查询

A) 添加相同的 ITEMNO，其“UNIT”为“CT”，且 CONVERSION 与“CTN”相同

B) 如果 ITEMNO 已经有一个“CT”的“UNIT”，则使用与“CTN”相同的 CONVERSION 更新它

除了这两个“单位”之外，还有这些是我唯一需要玩的。

尝试将此作为更新，但没有奏效:(

update ICUNIT 
set ICUNIT.CONVERSION  = (
select CONVERSION from ICUNIT
where UNIT = 'CTN')
from ICUNIT
where UNIT = 'CT'

希望有人能帮忙！！

Answer 1

正如评论中提到的，我建议避开MERGE。这些文章中有很多背景：

• Use Caution with SQL Server's MERGE Statement
• "UPSERT" Race Condition With MERGE
• Mythbusting: Concurrent Update/Insert Solutions
• Be Careful with the Merge Statement
• What To Avoid If You Want To Use MERGE

您需要同时执行INSERT 和UPDATE。这是两个需要在受保护事务中进行的操作，即使您使用像 MERGE 这样的语法糖，它看起来就像一个操作。

BEGIN TRANSACTION;

UPDATE trg WITH (UPDLOCK, SERIALIZABLE) 
  SET trg.CONVERSION = src.CONVERSION,
    ... other columns you want to match ...
FROM dbo.ICUNIT AS src
INNER JOIN dbo.ICUNIT AS trg
  ON src.ITEMNO = trg.ITEMNO
  WHERE src.CONVERSION = 'CTN'
    AND trg.CONVERSION = 'CT';

INSERT dbo.ICUNIT(ITEMNO, CONVERSION, ... other columns ...)
  SELECT ITEMNO, CONVERSION = 'CT', ... other columns ...
    FROM dbo.ICUNIT AS src
    WHERE src.CONVERSION = 'CTN'
      AND NOT EXISTS (SELECT 1 FROM dbo.ICUNIT AS trg
        WHERE trg.CONVERSION = 'CT' AND trg.ITEMNO = src.ITEMNO);

COMMIT TRANSACTION;

它比MERGE 多了几个字符，但避免了它的所有固有问题（我在Please stop using this UPSERT anti-pattern 中解释了更多）。如果您坚持使用MERGE，请，请，请您将HOLDLOCK 应用于目标表 - 如果您不这样做，那就是它背后的多个操作的事实真正开始解开。

无论如何，这似乎是一个奇怪的目标...如果您希望两行始终匹配除CONVERSION 以外的所有列，为什么还要设置两行？

Answer 2

听起来你想要MERGE

WITH
  t AS (SELECT * FROM ICUNIT WHERE CONVERSION = 'CT' ),
  s AS (SELECT * FROM ICUNIT WHERE CONVERSION = 'CTN')
MERGE t WITH (HOLDLOCK)
USING s
ON t.ITEMNO = s.ITEMNO
WHEN MATCHED THEN
  UPDATE SET
    CONVERSION = s.CONVERSION,
    AUDTDATE = s.AUDTDATE,
    AUDTTIME = s.AUDTTIME,
    AUDTUSER = s.AUDTUSER,
    AUDTORG = s.AUDTORG
WHEN NOT MATCHED THEN INSERT
  (ITEMNO, UNIT, CONVERSION,AUDTDATE,AUDTTIME,AUDTUSER,AUDTORG)
  VALUES
  (s.ITEMNO, 'CT', s.CONVERSION,s.AUDTDATE,s.AUDTTIME,s.AUDTUSER,s.AUDTORG);