Once you've defined the nature of the query in a Query object you can then use QueryTask to execute the query.
First, create an instance of the QueryTask object. You create a QueryTask object by passing the URL of the layer against which the query will be executed to its constructor. Note from the code example that follows that the map service URL includes an index number which references a specific layer in the map service which will be queried:
myQueryTask = new
QueryTask("http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_CENSUS_USA/MapServer/5"); Once you have created the QueryTask object you can perform a query against its layer passing a Query object to its QueryTask.execute() method.
QueryTask.execute() accepts three parameters: the Query object, a function to call when the operation succeeds, and a function to call if the operation fails.
The syntax for QueryTask.execute() is provided as follows:
QueryTask.execute(objQuery...