You can horizontally partition tables through views in SQL Server 7.0, which can provide I/O performance benefits when database users query subsections of large views. For example, if a large table documents sales for all sales departments for a year and if retrievals from this table are based on a single sales department, a partitioned view can be employed. A sales table is defined for each sales department, a constraint is defined on the sales department column on each table, and then a view is created on all of the tables to form a partitioned view. The query optimizer uses the constraint on the sales department column. When the view is queried, all of the sales department tables that do not match the sales department value provided in the query are ignored by the query optimizer and no I/O is performed against those base tables. This improves query performance by reducing I/O. For more information, see SQL Server Books Online.