CHANGETABLE (Transact-SQL)

Artikkeli
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Returns change tracking information for a table. You can use this statement to return all changes for a table or change tracking information for a specific row.

Transact-SQL syntax conventions

Syntax

CHANGETABLE (  
    { CHANGES <table_name> , <last_sync_version> 
    | VERSION <table_name> , <primary_key_values> } 
    , [ FORCESEEK ] 
    )  
[AS] <table_alias> [ ( <column_alias> [ ,...n ] )  
  
<primary_key_values> ::=  
( <column_name> [ , ...n ] ) , ( <value> [ , ...n ] )

Arguments

CHANGES table_name , last_sync_version
Returns tracking information for all changes to a table that have occurred since the version that is specified by last_sync_version.

table_name
Is the user-defined table on which to obtain tracked changes. Change tracking must be enabled on the table. A one-, two-, three-, or four-part table name can be used. The table name can be a synonym to the table.

last_sync_version
A nullable bigint scalar value. An expression will cause a syntax error. If the value is NULL, all tracked changes are returned. When it obtains changes, the calling application must specify the point from which changes are required. The last_sync_version specifies that point. The function returns information for all rows that have been changed since that version. The application is querying to receive changes with a version greater than last_sync_version. Typically, before it obtains changes, the application will call CHANGE_TRACKING_CURRENT_VERSION() to obtain the version that will be used the next time changes are required. Therefore, the application does not have to interpret or understand the actual value. Because last_sync_version is obtained by the calling application, the application has to persist the value. If the application loses this value then it will need to re-initialize data. last_sync_version should be validated to ensure that it is not too old, because some or all the change information might have been cleaned up according to the retention period configured for the database. For more information, see CHANGE_TRACKING_MIN_VALID_VERSION (Transact-SQL) and ALTER DATABASE SET Options (Transact-SQL).

VERSION table_name, { primary_key_values }
Returns the latest change tracking information for a specified row. Primary key values must identify the row. primary_key_values identifies the primary key columns and specifies the values. The primary key column names can be specified in any order.

table_name
Is the user-defined table on which to obtain change tracking information. Change tracking must be enabled on the table. A one-, two-, three-, or four-part table name can be used. The table name can be a synonym to the table.

column_name
Specifies the name of primary key column or columns. Multiple column names can be specified in any order.

value
Is the value of the primary key. If there are multiple primary key columns, the values must be specified in the same order as the columns appear in the column_name list.

[ FORCESEEK ]
Applies to: SQL Server (Starting with SQL Server 2016 (13.x) SP2 CU16, SQL Server 2017 (14.x) CU24, and SQL Server 2019 (15.x) CU11), Azure SQL Database, and Azure SQL Managed Instance

Optional parameter that forces a seek operation to be used to access the table_name. In some cases where very few rows have changed, a scan operation may still be used to access the table_name. If a scan operation this introduces a performance issue, use the FORCESEEK parameter.

[AS] table_alias [ (column_alias [ ,...n ] ) ]
Provides names for the results that are returned by CHANGETABLE.

table_alias
Is the alias name of the table that is returned by CHANGETABLE. table_alias is required and must be a valid identifier.

column_alias
Is an optional column alias or list of column aliases for the columns that are returned by CHANGETABLE. This enables column names to be customized in case there are duplicate names in the results.

Return Types

table

Return Values

CHANGETABLE CHANGES

When CHANGES is specified, zero or more rows that have the following columns are returned.

Column name	Data type	Description
SYS_CHANGE_VERSION	bigint	Version value that is associated with the last change to the row
SYS_CHANGE_CREATION_VERSION	bigint	Version values that are associated with the last insert operation.
SYS_CHANGE_OPERATION	nchar(1)	Specifies the type of change: U = Update I = Insert D = Delete
SYS_CHANGE_COLUMNS	varbinary(4100)	Lists the columns that have changed since the last_sync_version (the baseline). Note that computed columns are never listed as changed. The value is NULL when any one of the following conditions is true: Column change tracking is not enabled. The operation is an insert or delete operation. All nonprimary key columns were updated in one operation. This binary value should not be interpreted directly. Instead, to interpret it, use CHANGE_TRACKING_IS_COLUMN_IN_MASK().
SYS_CHANGE_CONTEXT	varbinary(128)	Change context information that you can optionally specify by using the WITH clause as part of an INSERT, UPDATE, or DELETE statement.
<primary key column value>	Same as the user table columns	The primary key values for the tracked table. These values uniquely identify each row in the user table.

CHANGETABLE VERSION

When VERSION is specified, one row that has the following columns is returned.

Column name	Data type	Description
SYS_CHANGE_VERSION	bigint	Current change version value that is associated with the row. The value is NULL if a change has not been made for a period longer than the change tracking retention period, or the row has not been changed since change tracking was enabled.
SYS_CHANGE_CONTEXT	varbinary(128)	Change context information that you can optionally specify by using the WITH clause as part of an INSERT, UPDATE, or DELETE statement.
<primary key column value>	Same as the user table columns	The primary key values for the tracked table. These values uniquely identify each row in the user table.

Remarks

The CHANGETABLE function is typically used in the FROM clause of a query as if it were a table.

CHANGETABLE(CHANGES...)

To obtain row data for new or modified rows, join the result set to the user table by using the primary key columns. Only one row is returned for each row in the user table that has been changed, even if there have been multiple changes to the same row since the last_sync_version value.

Primary key column changes are never marked as updates. If a primary key value changes, it is considered to be a delete of the old value and an insert of the new value.

If you delete a row and then insert a row that has the old primary key, the change is seen as an update to all columns in the row.

The values that are returned for the SYS_CHANGE_OPERATION and SYS_CHANGE_COLUMNS columns are relative to the baseline (last_sync_version) that is specified. For example, if an insert operation was made at version 10 and an update operation at version 15, and if the baseline last_sync_version is 12, an update will be reported. If the last_sync_version value is 8, an insert will be reported. SYS_CHANGE_COLUMNS will never report computed columns as having been updated.

Generally, all operations that insert, update, or delete of data in user tables are tracked, including the MERGE statement.

The following operations that affect user table data are not tracked:

Executing the UPDATETEXT statement. This statement is deprecated and will be removed in a future version of SQL Server. However, changes that are made by using the .WRITE clause of the UPDATE statement are tracked.
Deleting rows by using TRUNCATE TABLE. When a table is truncated, the change tracking version information that is associated with the table is reset as if change tracking has just been enabled on the table. A client application should always validate its last synchronized version. The validation fails if the table has been truncated.

CHANGETABLE(VERSION...)

An empty result set is returned if a nonexistent primary key is specified.

The value of SYS_CHANGE_VERSION might be NULL if a change has not been made for longer than the retention period (for example, the cleanup has removed the change information) or the row has never been changed since change tracking was enabled for the table.

Permissions

Requires the SELECT permission on the primary key columns and VIEW CHANGE TRACKING permission on the table that is specified by the <table_name> value to obtain change tracking information.

Examples

A. Returning rows for an initial synchronization of data

The following example shows how to obtain data for an initial synchronization of the table data. The query returns all row data and their associated versions. You can then insert or add this data to the system that will contain the synchronized data.

-- Get all current rows with associated version  
SELECT e.[Emp ID], e.SSN, e.FirstName, e.LastName,  
    c.SYS_CHANGE_VERSION, c.SYS_CHANGE_CONTEXT  
FROM Employees AS e  
CROSS APPLY CHANGETABLE   
    (VERSION Employees, ([Emp ID], SSN), (e.[Emp ID], e.SSN)) AS c;

B. Listing all changes that were made since a specific version

The following example lists all changes that were made in a table since the specified version (@last_sync_version). [Emp ID] and SSN are columns in a composite primary key.

DECLARE @last_sync_version bigint;  
SET @last_sync_version = <value obtained from query>;  
SELECT [Emp ID], SSN,  
    SYS_CHANGE_VERSION, SYS_CHANGE_OPERATION,  
    SYS_CHANGE_COLUMNS, SYS_CHANGE_CONTEXT   
FROM CHANGETABLE (CHANGES Employees, @last_sync_version) AS C;

C. Obtaining all changed data for a synchronization

The following example shows how you can obtain all data that has changed. This query joins the change tracking information with the user table so that user table information is returned. A LEFT OUTER JOIN is used so that a row is returned for deleted rows.

-- Get all changes (inserts, updates, deletes)  
DECLARE @last_sync_version bigint;  
SET @last_sync_version = <value obtained from query>;  
SELECT e.FirstName, e.LastName, c.[Emp ID], c.SSN,  
    c.SYS_CHANGE_VERSION, c.SYS_CHANGE_OPERATION,  
    c.SYS_CHANGE_COLUMNS, c.SYS_CHANGE_CONTEXT   
FROM CHANGETABLE (CHANGES Employees, @last_sync_version) AS c  
    LEFT OUTER JOIN Employees AS e  
        ON e.[Emp ID] = c.[Emp ID] AND e.SSN = c.SSN;

D. Detecting conflicts by using CHANGETABLE(VERSION...)

The following example shows how to update a row only if the row has not changed since the last synchronization. The version number of the specific row is obtained by using CHANGETABLE. If the row has been updated, changes are not made and the query returns information about the most recent change to the row.

-- @last_sync_version must be set to a valid value  
UPDATE  
    SalesLT.Product  
SET  
    ListPrice = @new_listprice  
FROM  
    SalesLT.Product AS P  
WHERE  
    ProductID = @product_id AND  
    @last_sync_version >= ISNULL (  
        (SELECT CT.SYS_CHANGE_VERSION FROM   
            CHANGETABLE(VERSION SalesLT.Product,  
            (ProductID), (P.ProductID)) AS CT),  
        0);

Jaa