如何处理合并SQL的WHEN MATCHED中的空值？

Question

MERGE TARGET T
USING SOURCE S
ON T.ID=S.ID

WHEN MATCHED AND (S.NAME<>T.NAME OR S.DOB<>T.DOB)
THEN UPDATE
SET T.NAME=S.NAME, T.DOB=S.DOB;

以上无法处理源端或目标端 name/dob 为空的情况。因为与 NULL 的比较返回 false。这要怎么处理呢？

例子：

COALESCE(S.NAME,'')<>COALESCE(T.NAME,'')

或者

COALESCE(S.NAME,0)<>COALESCE(T.NAME,0)

或者

NULLIF(S.NAME,'')<>NULLIF(T.NAME,'')

Answer 1

• 用于EXISTS( SELECT src.* EXCEPT SELECT tgt.* )检测语句中源行是否与目标行不同MERGE INTO。
• 在 T-SQL 中， and确实EXCEPT 能成功地比较NULL值， andNULL与NOT NULL值，而（正如您所注意到的）=and<>运算符却不能。
  • IS DISTINCT FROM另一种选择是对每一列使用，但是 SQL Server 还不支持此功能。
• 不要忘记还指定WITH HOLDLOCK. 与 非常重要MERGE。

就像这样：

MERGE INTO destTable WITH (HOLDLOCK) AS tgt
USING srcRelation AS src ON
    /* Only specify PK columns here: */
    tgt.Pk1 = src.Pk1
    AND
    tgt.Pk2 = src.Pk2

WHEN MATCHED AND EXISTS (

    /*
    Only specify "data" columns here; these are the columns which are also included in the UPDATE SET clause below.
    Don't include GENERATED, rowversion, PRIMARY KEY, and other "non-data" columns
    */

    SELECT tgt.Col1, tgt.Col2, tgt.Col3, etc
    EXCEPT
    SELECT src.Col1, src.Col2, src.Col3, etc
)
THEN UPDATE SET
    tgt.Col1 = src.Col1,
    tgt.Col2 = src.Col2,
    tgt.Col3 = src.Col3

WHEN NOT MATCHED BY TARGET THEN etc...;

具体来说，就您的情况而言：

MERGE INTO "target" WITH (HOLDLOCK) AS tgt
USING "source" AS src ON
    tgt."ID" = src."ID"
WHEN MATCHED AND EXISTS (
    SELECT tgt."Name"
    EXCEPT
    SELECT src."Name"
)
THEN UPDATE SET
    tgt."Name" = src."Name";