# Introduction

[L|W]izard of Ex

This tutorial will show you how to use **filter expressions in FME** in order to query OGC Web Feature Services (**WFS**). The relevant **Filter Encoding Standard** is developed by OGC and ISO TC/211. This standard describes a key value pair (KVP) system using XML documents. FME supports all versions of WFS, i.e. versions 1.x, 2.x and 3.x also known as OGC API - FEATURES. So why could it be necessary to use filter expressions? What is the benefit comparing to FME standard functionality? The answer is: most WFS services are limited to a certain number of features, normally to 10.000 or 30.000 features. Imagine you want to use a WFS delivering cadastral parcels and you only need one or a small number of cadastral parcel features. Let's say you're using the cadastral parcel WFS of North Rhine-Westphalia which actually holds available more than 9.000.000 features. How to get the one feature with cadastral parcel number "05430202600946\_\_\_\_\_\_"? One possible strategy could be to download the whole bunch to your local hard drive first (good luck!) and to do the selection in a second step for example using FMEs TestFilter transformer. A more effective way could be to ask the WFS for the concrete features you need. This saves time, network traffic and local disk space. Asking a system for a subset of data ... this reminds you of something known? Yes, you're right! Think about **SQL** (**S**tructured **Q**uery **L**anguage). SQL is a standard language for storing, manipulating and retrieving data in databases. To query a database for a concrete dataset you'll use the key value pair "attribute = value", e.g. ```plsql SELECT * FROM cadastral_parcels WHERE parcel_number = '05430202600946______' ; ``` Filter expressions are working the same way: they use key value pairs. These keys can be: 1. attributes 2. spatial operators 3. a combination of both (attributive and spatial) The difference to SQL is: filter expressions are written in XML (Don: we still love XML!). A key value pair may look like this: ```xml parcel_number 05430202600946______ ``` You may wonder about how this could work? Something is missing ... Indeed: we need an operator or function like the equal sign "=" in SQL. 
<fes:PropertyIsEqualTo>   
   <fes:ValueReference>parcel_number</fes:ValueReference>
   <fes:Literal>05430202600946______</fes:Literal>
</fes:PropertyIsEqualTo>
*PropertyIsEqualTo* is the equivalent to "*=*" in SQL. But before we're diving into the deep, I've to express my gratitude to some people. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://olivers-expression-lab.gitbook.io/filter-expressions-in-fme/introduction.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.