In a very simple case your project may consist of only one table with a few transformations applied. However, with the help of derived tables you can build much more complex transformation logic. Derived tables is a cornerstone feature of EasyMorph. It is used in vast majority of projects. You can see them in all examples.
Derived tables are tables that are created not by loading data from a file or database, but by replicating a final state of some existing table. In another words, a derived table is a dynamic replica of another table. Dynamic in the sense, that when a source table changes, all tables derived from it also change. There can be many tables derived from one table. A derived table is a regular table that starts from "Derive table" transformation. A derived table can be a source for another one or more derived tables.
In the example below, a dataset is aggregated in two different ways, using two derived tables. In one derived table the dataset is aggregated by city, in the other — by year.
To derive a new table right-click a table header and choose Derive > Table, or press Ctrl+D. There are several typical cases that are arranged with the help of derived tables:
In the example below, table rows are filtered, and only products with sales less than average are kept. Table "Avg.Sales" is a derived helper table that is used to calculate the average sales amount. The Peek transformation (Step 2) appends the average sales amount from one table to another as a new column. Then in Step 3 table rows where Sales < Avg.Sales are filtered.
Sometimes a table needs to be transformed using a (possibly modified) copy of itself, e.g. for a self-join. Referencing in a table the table itself, or one of its derived tables would lead to a circular reference and therefore is not allowed. To avoid circular referencing create one more derived table. Tables derived from the same parent table are independent, and therefore can be referenced withough creating a circular dependency. In the example below a table is appended with a slightly modified version of itself.