Let's create an HTTP imposter with multiple stubs:

POST /imposters HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json
Content-Type: application/json

{
  "port": 4556,
  "protocol": "http",
  "stubs": [
    {
      "responses": [{ "is": { "body": "first" } }],
      "predicates": [{
        "deepEquals": {
          "query": {}
        }
      }]
    },
    {
      "responses": [{ "is": { "body": "second" } }],
      "predicates": [{
        "deepEquals": {
          "query": {
            "first": "1"
          }
        }
      }]
    },
    {
      "responses": [{ "is": { "body": "third" } }],
      "predicates": [{
        "deepEquals": {
          "query": {
            "first": "1",
            "second": "2"
          }
        }
      }]
    }
  ]
}

The first predicate matches only a request without a querystring.

GET /test HTTP/1.1
Host: localhost:4556

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

first

The second stub matches only if the exact querystring is sent.

GET /test?First=1 HTTP/1.1
Host: localhost:4556

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

second

The third stub matches only if both query keys are sent.

GET /test?Second=2&First=1 HTTP/1.1
Host: localhost:4556

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

third

Any additional query parameters will trigger the default HTTP response.

GET /test?Second=2&First=1&Third=3 HTTP/1.1
Host: localhost:4556

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked