Guaranteeing Uniqueness in Updating DataSet to Database

This section describes how the OracleDataAdapter object configures the PrimaryKey and Constraints properties of the DataTable object which guarantee uniqueness when the OracleCommandBuilder object is updating DataSet changes to the database.

Using the OracleCommandBuilder object to dynamically generate DML statements to be executed against the database is one of the ways to reconcile changes made in a single DataTable object with the database.

In this process, the OracleCommandBuilder object must not be allowed to generate DML statements that may affect (update or delete) more that a single row in the database when reconciling a single DataRow change. Otherwise the OracleCommandBuilder could corrupt data in the database.

To guarantee that each DataRow object change affects only a single row, there must be a set of DataColumn objects in the DataTable for which all rows in the DataTable have a unique set of values. The set of DataColumn objects indicated by the properties DataTable.PrimaryKey and DataTable.Constraints meets this requirement. The OracleCommandBuilder object determines uniqueness in the DataTable by checking if the DataTable.PrimaryKey is not a null value or if there exists a UniqueConstraint object in the DataTable.Constraints collection.

This discussion first explains what constitutes uniqueness in DataRow objects and then explains how to maintain that uniqueness while updating, through the DataTable property configuration.

This section includes the following topics:

• What Constitutes Uniqueness in DataRow Objects?

• Configuring PrimaryKey and Constraints Properties

• Updating Without PrimaryKey and Constraints Configuration

What Constitutes Uniqueness in DataRow Objects?

This section describes the minimal conditions that must be met to guarantee uniqueness of DataRow objects. The condition of uniqueness must be guaranteed before the DataTable.PrimaryKey and DataTable.Constraints properties can be configured, as described in the next section.

Uniqueness is guaranteed in a DataTable object if any one of the following is true:

• All the columns of the primary key are in the select list of the OracleDataAdapter.SelectCommand property.

• All the columns of a unique constraint are in the select list of the OracleDataAdapter.SelectCommand property, with at least one involved column having a NOT NULL constraint defined on it.

• All the columns of a unique index are in the select list of the OracleDataAdapter.SelectCommand property, with at least one of the involved columns having a NOT NULL constraint defined on it.

• A ROWID is present in the select list of the OracleDataAdapter.SelectCommand property.

Note:

A set of columns, on which a unique constraint has been defined or a unique index has been created, requires at least one column that cannot be null for the following reason: if all the columns of the column set can be null, then multiple rows could exist that have a NULL value for each column in the column set. This would violate the uniqueness condition that each row has a unique set of values for the column set.

Configuring PrimaryKey and Constraints Properties

If the minimal conditions described in "What Constitutes Uniqueness in DataRow Objects?" are met, then the DataTable.PrimaryKey or DataTable.Constraints properties can be set.

After these properties are set, the OracleCommandBuilder object can determine uniqueness in the DataTable by checking the DataTable.PrimaryKey property or the presence of a UniqueConstraint object in the DataTable.Constraints collection. Once uniqueness is determined, the OracleCommandBuilder object can safely generate DML statements to update the database.

The OracleDataAdapter.FillSchema method attempts to set these properties according to this order of priority:

1. If the primary key is returned in the select list, it is set as the DataTable.PrimaryKey property.

2. If a set of columns that meets the following criteria is returned in the select list, it is set as the DataTable.PrimaryKey property.

   Criteria: The set of columns has a unique constraint defined on it or a unique index created on it, with each column having a NOT NULL constraint defined on it.

3. If a set of columns that meets the following criteria is returned in the select list, a UniqueConstraint object is added to the DataTable.Constraints collection, but the DataTable.PrimaryKey property is not set.

   Criteria: The set of columns has a unique constraint defined on it or a unique index created on it, with at least one column having a NOT NULL constraint defined on it.

4. If a ROWID is part of the select list, it is set as the DataTable.PrimaryKey property.

Additionally, the OracleDataAdapter.FillSchema method performs as follows:

• Setting the DataTable.PrimaryKey property implicitly creates a UniqueConstraint object.

• If a column is part of the DataTable.PrimaryKey property or the UniqueConstraint object, or both, it will be repeated for each occurrence of the column in the select list.

Updating Without PrimaryKey and Constraints Configuration

If the DataTable.PrimaryKey or Constraints properties have not been configured, for example, if the application has not called the OracleDataAdapter.FillSchema method, the OracleCommandBuilder object directly checks the select list of the OracleDataAdapter.SelectCommand property to determine if it guarantees uniqueness in the DataTable. However this check results in a database round-trip to retrieve the metadata for the SELECT statement of the OracleDataAdapter.SelectCommand.

Note that OracleCommandBuilder object cannot update a DataTable created from PL/SQL statements because they do not return any key information in their metadata.