We will model cities as nodes, as shown in Figure 2.4, with the city's name and country as properties. Cities should have unique names. For this, we can add a constraint before we start creating cities in our graph.

The query is as follows:

neo4j-sh (?)$ CREATE CONSTRAINT ON (city: City) ASSERT city.name IS UNIQUE;

The output of the preceding query is as shown:

+-------------------+
| No data returned. |
+-------------------+
Constraints added: 1

This adds a constraint on all nodes that will henceforth be created with the label City to have a unique name property. The city in (city: City) is a placeholder, like a variable, for any node with label City. Note that the addition of a uniqueness constraint is idempotent—it can be repeated multiple times without throwing an error or changing the constraint after it first gets added.

It's a good practice to add a uniqueness constraint, like we did, before we start adding nodes that have a particular label. While...