What is the namespace in which .NET have the data functionality classes?

System.data :

This namespace contains the basic objects used for accessing &  storing relational data, like  DataTable,DataSet, and DataRelation. Each of these is independent of the type of the data source and the way we connect to it.

System.Data.OleDB :

This namespace contains the objects that we use to connect to a data source via an OLE-DB provider, like the OleDbCommand, OleDbConnection,  etc. These objects inherit from the common base classes,&  so have the similar methods,properties,  and events as the SqlClient equivalents.

System.Data.SqlClient :

This namespace contains the objects that we use to connect the  data source via the Tabular Data Stream (TDS) interface of  the Microsoft SQL Server . This can normally give better performance as it removes some of the intermediate layers required by an OLE-DB connection.

System.XML

This namespace Contains the basic objects needed to create, store, read, write, and for manipulating XML documents according to W3C recommendations.