如何在SQL Server複製中有效率地只檢索更新的欄位？

取得SQL Server 中複製的更新欄位

簡介

在SQL Server 資料庫之間複製資料時，有必要識別已修改的欄位。預設機制 COLUMNS_UPDATED 提供更新欄位的位元表示。但是，本文提供了另一種方法來檢索僅包含更新的列值的 XML 程式碼段。

自訂更新觸發器函數

要實現此目的，請建立自訂更新觸發函數如下：

CREATE TRIGGER DBCustomers_Insert
ON DBCustomers
AFTER UPDATE
AS
BEGIN
    DECLARE @sql as NVARCHAR(1024);
    SET @sql = 'SELECT ';

    WITH UpdatedColumns AS (
        SELECT c.name AS ColumnName
        FROM sys.columns AS c
        JOIN inserted AS i ON c.object_id = i.object_id
        WHERE c.is_identity = 0
            AND i.Original_value  i.value
    )
    SELECT @sql = @sql + ',' + ColumnName
    FROM UpdatedColumns

    SET @sql = $sql + ' FROM inserted FOR XML RAW';

    DECLARE @x as XML;
    SET @x = CAST(EXEC(@sql) AS XML);

    .. use @x

END

替代解決方案，無需COLUMNS_UPDATED

代替COLUMNS_UPDATED，另一種方法包括取消透視插入和刪除的表、連接它們以及過濾任何更改：

CREATE TRIGGER TriggerName ON dbo.Sample_Table FOR DELETE, INSERT, UPDATE AS
BEGIN
    SET NOCOUNT ON;

    WITH deleted_unpvt AS (
        SELECT ContactID, FieldName, FieldValue
        FROM 
           (SELECT ContactID
                , cast(Forename as sql_variant) Forename
                , cast(Surname as sql_variant) Surname
                , cast(Extn as sql_variant) Extn
                , cast(Email as sql_variant) Email
                , cast(Age as sql_variant) Age
           FROM deleted) p
        UNPIVOT
           (FieldValue FOR FieldName IN 
              (Forename, Surname, Extn, Email, Age)
        ) AS deleted_unpvt
    ),
    inserted_unpvt AS (
        SELECT ContactID, FieldName, FieldValue
        FROM 
           (SELECT ContactID
                , cast(Forename as sql_variant) Forename
                , cast(Surname as sql_variant) Surname
                , cast(Extn as sql_variant) Extn
                , cast(Email as sql_variant) Email
                , cast(Age as sql_variant) Age
           FROM inserted) p
        UNPIVOT
           (FieldValue FOR FieldName IN 
              (Forename, Surname, Extn, Email, Age)
        ) AS inserted_unpvt
    )

    INSERT INTO Sample_Table_Changes (ContactID, FieldName, FieldValueWas, FieldValueIs)
    SELECT Coalesce (D.ContactID, I.ContactID) ContactID
        , Coalesce (D.FieldName, I.FieldName) FieldName
        , D.FieldValue as FieldValueWas
        , I.FieldValue AS FieldValueIs 
    FROM 
        deleted_unpvt d

            FULL OUTER JOIN 
        inserted_unpvt i
            on      D.ContactID = I.ContactID 
                AND D.FieldName = I.FieldName
    WHERE
         D.FieldValue  I.FieldValue --Changes
        OR (D.FieldValue IS NOT NULL AND I.FieldValue IS NULL) -- Deletions
        OR (D.FieldValue IS NULL AND I.FieldValue IS NOT NULL) -- Insertions
END

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