FDO uses expressions through its commands (including provider-specific commands) to specify input values in order to filter features. In general, commands in FDO do not support the SQL command language (the one exception is the optional SQLCommand). However, to facilitate ease of use for application developers, expressions in FDO can be specified using a textual notation that is based syntactically on expressions and SQL WHERE clauses. In FDO, expressions are not intended to work against tables and columns, but against feature classes, properties, and relationships. For example, an expression to select roads with four or more lanes might look like this:
Lanes >= 4
An expression is a construct that an application can use to build up a filter. In other words, an expression is a clause of a filter or larger expression. For example, “Lanes >=4 and PavementType = 'Asphalt'” takes two expressions and combines them to create a filter.