# Creating expressions

Compose allows you to create data transformations in several different places according to your needs. A transformation can either be a filter (i.e. excluding certain data) or an expression (i.e. manipulating a single record). The table below lists the places where transformations can be created and provides reasons for creating the transformation in each of the specified places.

Information note

Changes in a dimension expression or lookup of a column in a dimension are not updated retroactively. In order to update historical data, you would need to reload the data which could take a long time depending on the number of records and their history.

Data transformation location comparison

| Where the Transformation is Created | Reasons to Create a Transformation There | When the Transformation is Applied |
| --- | --- | --- |
| Replicate | <ul> <li>Filtering large amounts of data that is not needed for the data warehouse (in the present or the future)</li> <li>Obfuscation due to regulatory reasons or internal policies</li> <li>Data type conversion (e.g. converting a source data type that is not supported on the data warehouse platform)</li> </ul> | Before the data reaches the landing zone. |
| Model | <ul> <li>The default location if you are not sure where to put it</li> <li>General business logic</li> <li>Needed for several sources or several data marts</li> </ul> | Applied as an update to the staging tables after creating the mappings. |
| Data Warehouse | <ul> <li>Specific source preparation</li> <li>Needed for merging several sources</li> </ul> | Between the landing zone and the staging zone. |
| Data Mart | <ul> <li>Specific to a data mart</li> <li>Managed by a data mart data specialist</li> </ul> | Between the data warehouse and the data mart. |

See also [Defining reusable transformations](https://help.qlik.com/en-US/compose/December2024/Content/ComposeDWDL/Main/DW/Model/define_reusable_transformations.htm).

The following topics describe the Expression Builder:

* [Opening the expression builder](https://help.qlik.com/en-US/compose/December2024/Content/ComposeDWDL/Main/DW/Projects/tasks_expBuildOver.htm)
* [Expression builder overview](https://help.qlik.com/en-US/compose/December2024/Content/ComposeDWDL/Main/DW/Model/expression_builder_overview.htm)
* [Building expressions](https://help.qlik.com/en-US/compose/December2024/Content/ComposeDWDL/Main/DW/Model/tasks_buildExpress.htm)
* [Testing expressions](https://help.qlik.com/en-US/compose/December2024/Content/ComposeDWDL/Main/DW/Model/tasks_testExpress.htm)