1. A method for creating an expression to specify a subset of data, comprising: receiving a selection of a command; in response to receiving the selection of the command, displaying a natural language expression that includes the command and at least a first changeable field embedded in the displayed natural language expression; receiving a indication of first data for the first changeable embedded field from a direct interaction with the displayed natural language expression; modifying the natural language expression in response to the first data and displaying the modified natural language expression; displaying an add field indicator; receiving a selection of the add field indicator from a direct interaction with the displayed natural language expression; in response to the selection of the add field indicator, adding a second changeable field embedded in the natural language expression and displaying the natural language expression with the second changeable field embedded in the displayed natural language expression; receiving an indication of second data for the second changeable embedded field from a direct interaction with the displayed natural language expression; and modifying the natural language expression in response to the second data and displaying the modified natural language expression.