Perhaps the simplest form of query you can perform using Esri’s API is defining a layer’s definitionExpression. This is a property supported by a few different layer sub-classes, including FeatureLayer and ImageryLayer. If you’ve worked with desktop GIS, you’ve probably encountered this sort of feature. The idea is that you can restrict a layer’s underlying data to a subset meeting certain criteria. Using a definition expression is common when working with datasets that cover multiple political subdivisions (e.g., states in the U.S.).
The definitionExpression property can be set to any valid SQL where clause. Here is an example that filters out roughly half of the Jen & Barry's cities by selecting only the features whose UNIVERSITY value is 1.