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In Telerik Reporting the expressions are the main tool for describing the run-time behavior of the report. They are used to determine the number of records in the Data Source or the number of Group members. In Conditional Formatting rules filters are used to express the conditions on which the styling rules are applied. Filters are predicate expressions which should evaluate to True or False.
You can determine where you want to set a filter by the effect you want to achieve in your report. At run time, the report processor applies the defined filters in the following order: on the Data Source, and then on the DataItem, and then on Groups from the top down in each Group hierarchy. On a Crosstab, Table, and List, filters for row groups, column groups, and adjacent groups are applied independently. On a Graph, filters for CategoryGroups and SeriesGroups are applied independently. When the report processor calculates a filter, all filter equations are applied in the order they are defined in the Filters collection which is the equivalent of combining them with Boolean AND operations.
The idea behind filtering the retrieved data source records is the same as in the WHERE clause in a SQL statement. However there are two ways to achieve this effect:
"Server side" filtering
The most efficient way to save bandwidth is to filter data records before they are retrieved by the DataSource component. This is called filtering on the “server side”. To accomplish this you can use parameterized DataSource component for each DataSource component that support this. The next step is to map each DataSource parameter to Report Parameters using expressions. In this approach no Filters are needed. If however the data source does not support parameters, or you must run stored procedures and cannot modify the query use report filters.
"Client side" filtering
A filter can be added to the Filters collection on a Report, a DataItem level or in the Report Parameter’s AvailableValues. Filters are applied to the data set after it is retrieved by the DataSource. This method of filtering is called "client side" filtering and should be avoided for big data. Usually the Filter expressions compare fields of the data against report parameters or master data fields. These filters are calculated over the entire set of data and should not use the aggregate functions for the Expressions.
Set a filter on a Group when you want to include or exclude certain members for a Group. These filters operate over the group members after the groups are calculated and can contain aggregate functions. Group filters are similar to the HAVING clause in a SQL statement.
Filters applied to Report Groups will control the visibility of the Group members and GroupHeader/GroupFooter sections.
Filters applied to Crosstab, Table and List Row/Column Groups will change the visibility of corresponding rows and columns and also their headers.
For Graph Filters can be applied to SeriesGroups and CategoryGroups and will control the visibility of Series and Category members.
To apply the Style of report items dynamically Telerik Reporting uses Filter expressions. At run time, the report processor evaluates the filter expressions and substitutes the result for the property value, just as if you had set the property to a static value at design time. When the report is rendered, the run-time value is used. The filter expressions can contain aggregate functions.