Comparison Operators

The good ones go into the pot, the bad ones go into your crop

Remember Introduction, where I compared XML Filter Expressions with SQL.

Investigating WFS capabilities in GetCapabilities shows us, which comparison operations are supported by the WFS, e.g.

<fes:ComparisonOperators>
   <fes:ComparisonOperator name="PropertyIsEqualTo"/>
   <fes:ComparisonOperator name="PropertyIsNotEqualTo"/>
   <fes:ComparisonOperator name="PropertyIsLessThan"/>
   <fes:ComparisonOperator name="PropertyIsGreaterThan"/>
   <fes:ComparisonOperator name="PropertyIsLessThanOrEqualTo"/>
   <fes:ComparisonOperator name="PropertyIsGreaterThanOrEqualTo"/>
   <fes:ComparisonOperator name="PropertyIsLike"/>
   <fes:ComparisonOperator name="PropertyIsNull"/>
   <fes:ComparisonOperator name="PropertyIsNil"/>
   <fes:ComparisonOperator name="PropertyIsBetween"/>
</fes:ComparisonOperators>

Now, we will have a look at some Comparison Operators. As they are easy to understand, we look only at a small selection.

To do so, we will use our traffic meter WFS again.

Parameter	Value
WFS URL	https://www.wfs.nrw.de/wfs/strassen_nrw?
Version	2.0.0
Features Types	Zaehlstellen (counting points)
Max Features	leave blank
xmlns	ms="http://mapserver.gis.umn.edu/mapserver"

PropertyIsEqualTo

To recap, remember the Filter Expression we already used

<fes:Filter 
   xmlns:fes="http://www.opengis.net/fes/2.0"
   xmlns:ms="http://mapserver.gis.umn.edu/mapserver">
   <fes:PropertyIsEqualTo>
      <fes:ValueReference>ms:ZSTNR</fes:ValueReference>
      <fes:Literal>52082215</fes:Literal>
   </fes:PropertyIsEqualTo>
</fes:Filter>

PropertyIsEqualTo works similar to the equal operator ("=") in SQL.

Any feature which exactly matches the Filter Expresssion is returned.

PropertyIsBetween

PropertyIsBetween defines a range of values for 1 attribute by a

1. LowerBoundary

2. UpperBoundary

<fes:Filter
	xmlns:fes="http://www.opengis.net/fes/2.0"
	xmlns:ms="http://mapserver.gis.umn.edu/mapserver">
	<fes:PropertyIsBetween>
		<fes:ValueReference>ms:ZSTNR</fes:ValueReference>
		<fes:LowerBoundary><fes:Literal>39000000</fes:Literal></fes:LowerBoundary>
		<fes:UpperBoundary><fes:Literal>40000000</fes:Literal></fes:UpperBoundary>
	</fes:PropertyIsBetween>
</fes:Filter>

As our traffic meter WFS uses ascending integers for the counting point number (ZSTNR), we are able to cut out a piece like this

PropertyIsLike - Wildcards

Wildcards are really useful! The PropertyIsLike operator allows you to do pattern matching.

In our traffic meter WFS the attribute "ERFASSART" describes the type of traffic meter in combination with year dates, e.g. "2017".

Let's select all traffic meters containing "2017" in ERFASSART, no matter of the position within the description.

<fes:Filter 
   xmlns:fes="http://www.opengis.net/fes/2.0"
   xmlns:ms="http://mapserver.gis.umn.edu/mapserver">
   <fes:PropertyIsLike wildCard='*' singleChar='.' escapeChar='!'>
      <fes:ValueReference>ms:ERFASSART</fes:ValueReference>
      <fes:Literal>*2017*</fes:Literal>
   </fes:PropertyIsLike>
</fes:Filter>

Let's try another example. This time parking decks in Hamburg.

Key	Value
WFS URL	https://qs-geodienste.hamburg.de/HH_WFS_Verkehr_opendata?SERVICE=WFS
Version	2.0.0
Features Types	verkehr_parkhaeuser (parking decks)
Max Features	leave blank
xmlns	de.hh.up="https://registry.gdi-de.org/id/de.hh.up"
	preise (de) = fees (en) kostenlos (de) = free of charge (de)

We want to select only those parking decks with no fees at least for the first 30 minutes.

<fes:Filter
	xmlns:fes="http://www.opengis.net/fes/2.0"
	xmlns:de.hh.up="https://registry.gdi-de.org/id/de.hh.up">
	<fes:PropertyIsLike wildCard='*' singleChar='.' escapeChar='!'>
		<fes:ValueReference>de.hh.up:preise</fes:ValueReference>
		<fes:Literal>*kostenlos*</fes:Literal>
	</fes:PropertyIsLike>
</fes:Filter>

Umlauts

If you struggle with umlauts (ÄÖÜäöüß) in Comparison Operators, please try wildcards (PropertyIsLike) instead!