SQLxD Part 20 — Transformers

This is the twentieth part of the SQLxD series. For your convenience you can find other parts in the table of contents in Part 1 – XML Transformation

We can extract nodes, transform them into rows, filter, join, group, order, and transform them. It is high time to glue all the things together in order to have engine working.

For connecting cells and expressions we will use transformers. They execute things in correct order and allow stacking of different operations.

We start with transformer for row. We can see that it transforms cells of row. We can implement identity transformer:

We can also implement proper row transformer:

Now cell transformer:

We can see that transformer takes expression and calculates result by using it. This is the thing which glues all other mechanisms together: we pass row to row transformer which uses cell expression to transform cell and return new row.

And now tests:

And we are ready to implement last operator: SELECT.