Dataset merge Method

Is the use of the merge method in msdn:

The merge method is used to merge two dataset objects with similar architectures. Merge applications on the clientProgramIt is usually used to merge the recent changes in the data source into the existing dataset. This allows client applications to have dataset refreshed with the latest data in the data source. The merge method is usually called at the end of a series of processes that involve verifying changes, removing errors, updating the data source using changes, and finally refreshing the existing dataset.
In client applications, there is usually a button that you can click to collect and verify the changed data, and then send it back to the middle layer component. In this case, the getchanges method is called first. This method returns another dataset optimized for verification and merging. The second DataSet object only contains the changed able and datarow objects. The result is a subset of the initial dataset. This subset is usually small, so it can be passed back to the middle layer components more efficiently. Then, the middle layer component updates the initial data source through the stored procedure using changes. Then, the middle layer can send back a new dataset, which contains the initial data and the latest data in the data source (by re-running the initial query ); or it can send back a subset that contains all changes made to it from the data source. (For example, if the data source automatically creates a unique primary key value, these values can be propagated back to the client application .) In which case, you can use the merge method to merge the returned dataset back to the initial dataset of the client application.
When a new source dataset is merged to the target, any source row with the datarowstate value of unchanged, modified, or deleted will match the target row with the same primary key value. The source row with the datarowstate Value Added matches the new target row with the same primary key value as the new source row.
 

According to the above instructions, we know thatMergeWhen merging two data sets, the method compares the primary key values of rows. In this way, when a new row is added to the dataset, no problem exists. If the row is modified and the primary key value is not modified, however, if you modify the value of the primary key when changing the row, the problem arises.......The following is an example:

InSQL ServerNextProductsThe table structure is as follows:

 

InSQL ServerNextProductsThe table structure is as follows:

 

Column name	Data Type	Description
Code	Nchar	ProductCode(Primary Key Column)
Name	Nvarchar	Production name
Unitprice	Numeric	Unit Price

 

In. NetUsed inXSDGenerate a correspondingProductsdata. XSDThe structure is as follows:

 

column name	data type	description
Code	string	product code ( primary key column )
name	string	product name
unitprice	decimal	product unit price

 

AccordingMsdnInstructions, we passed at the front-endProductsdataAfter adding or modifying data, when submitting background updates, the common practice is as follows:

 

// Create a new dataset to save the changes made to the primary Dataset

 

Productsdata datasetchanges;

 

Datasetchanges = (productsdata) (productsdata. getchanges ());

 

// Check for any changes

 

If (datasetchanges! = NULL ){

 

Try {

 

// Make some changes, so try to call the update Method

 

// And pass the dataset and any parameters to update the data source

 

Updatedatasource (datasetchanges );

 

Productsdata. Merge (datasetchanges );

 

Productsdata. acceptchanges ();

 

}

 

Catch (system. Exception eupdate ){

 

Throw eupdate;

 

}

 

}

 

The above code is based onVs. netBased on the above Code, we simulate adding a row of data to the dataset and updating it:

 

Code	Name	Unitprice
1001	Jinsha chocolate	120.00

 

No problem. Let's modify this line of data and then update it.CodeChange1002After the update, the result data does not show the original line as we expected.CodeColumn Value1001Change1002, But added a row:

 

Code	Name	Unitprice
1001	Jinsha chocolate	120.00
1002	Jinsha chocolate	130.00

 

Note: In general, we seldom change the primary key value, but when the code is not used, it is generally allowed to changeCodeEspecially in the system implementation phase.

 

The reason for the above problems is not surprising.MergeThis is also true for the principle of the method, but this is the result we don't want. How can we solve this problem? Do you not allow users to change the primary key, but it does not seem to conform to the actual situation. How can this problem be solved?

 

To useMergeMethod, it only depends onMergeThe principle of merging data is as follows. If we do not want to change the primary key, we will not change the primary key.ProductsdataAdd an additional primary key to the dataset.IDColumn, andAutoincrementSetTrueTo convert the original primary key columnCodeChangeKeyColumn, as for the backgroundSQL ServerThe source table is not modified. The modification is as follows:

 

Column name	Data Type	Description
ID	Int	Auto-increment column (primary key)
Code	String	Product Code (Key)
Name	String	Production name
Unitprice	Decimal	Unit Price

 

After this changeIDIs an auto-incrementing column. We do not modify its value, so we can change it at will.CodeColumn value,MergeBecause the method is compared based on the ID Example when merging data, there will no longer be the problem of adding multiple rows in front.