Namespaces used to access database are

System.Data – It Contains all generic data access classes

System.Data.Common –It Contains classes which are shared / overridden by data providers

System.Data.OleDb - System.Data.OleDb provider classes are used to access database such as Oracle, MySQL, SQL Server and MS Access.

System.Data.SqlClient – It Contains classes for SQL Server

System.Data.SqlTypes – It Contains SQL Server data types