Practical GIS

SQL in GIS

SQL is the query language of relational databases. Traditionally, it was developed to help make easy and powerful queries on relational tables. As attribute data can be considered tabular, its power for creating intuitive queries on vector layers is unquestionable. No matter if some modern GIS software uses an object-oriented structure for working with vector data internally, the tradition of using SQL, or near-SQL syntax for creating simple queries has survived. This language is very simple at its core, thus, it is easy to learn and understand. A simple query on a database looks like the following:

    SELECT * FROM table WHERE column = value;

It uses the * wildcard for selecting everything from the table named table, where the ...