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Thematic query
Thematic 
queries result in the selection of thematic
information. This process is comparable to a
query of a conventional database, whose data have no spatial reference.
Query language
In a GIS, queries are performed either with SQL
or internal query languages. In a standard GIS the commands are directly entered; in contrast,
in a desktop GIS, the commands have to be selected using a dialogue system. The functionalities
of SQL are explained in the "Database Management and Systems Module"
(lesson 4: Structured Query Language SQL).
In the following examples, some user‐friendly interfaces for SQL formulations of thematic queries are
shown. With a few clicks the desired SQL commands are compiled. Behind the interface, the
SQL syntax is used to query the database.
Example 1: ArcView Interface
Example 2: ArcGIS Interface 
Query operators
A particular feature that distinguishes a GIS is the
possibility to query specific thematic information
about selected objects. The thematic query relies on the analysis of technical data (attribute data). The
query is performed using adequate selecting operators. In the following, three categories of
operators are presented:
- Relational operators: Besides the equal sign, relational operators can be used to formulate queries as well.
- Arithmetic operators: These operators are used for numeric attributes. E.g. there is the possibility to calculate the mean of an attribute or the sum of attribute values from a series of objects.
- Logic operators: Conditions are formulated with logic operators. The semantics (meaning) of these operators are similar to the meaning "AND", "OR" etc.
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