Is it possible to dynamically construct the SQL query where clause in ArcGIS layer based on the URL parameters?

if you're using javascript, you can create a query from your feature layer https://developers.arcgis.com/javascript/latest/api-reference/esri-layers-FeatureLayer.html#createQuery

var layer = new FeatureLayer({
  url: "https://baseURL/myMapServer/0"
});

var queryParams = layer.createQuery();
queryParams.where = "category = 'cat1'";

layer.queryFeatures(queryParams).then(function(res){
  ...
});

or if you want to build the URL : https://baseURL/myMapServer/0/query?where=category%3D'cat1'

What are you using on your local server?

What you have is a GET request. With PHP for example, you need to do something along the lines of:

$cat = $_GET['category'];

This will assign the value - in your example of "cat1" - to the variable $cat. You then proceed with building your SQL query and adding the $cat variable at the appropriate location.

Anyway, you need to look into how to grab the value of your GET request. I hope this helps.

This is a very ambiguous request. If I understand correctly you need to extract info from a call (GET request) to ArcGIS Server's REST endpoint, so you already have the complete URL (generated from some app perhaps). In this case:

//example URL: http://baseURL/arcgis/rest/services/myMapServer/0/query?where=NAME+like+%27%25test%25%27+AND+OBJECTID+%3C+1000&f=html
const mySearchParams = new URLSearchParams(window.location.search)
for (const [key, value] of mySearchParams) { do what you want with them ... }

If you want to you can only get the where param:

mySearchParams.get('where')
//"NAME like '%test%' AND OBJECTID < 1000"

Read here:

https://davidwalsh.name/query-string-javascript

https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams

Otherwise take a look at the ArcGIS Server REST API, more specifically the Query operation on a Map/Feature Service/Layer: https://developers.arcgis.com/rest/services-reference/query-map-service-layer-.htm You can play with the web UI, there is a where clause you can apply as well. The bad part is all other params get included in the GET request with no values. Another VERY BAD "feature" of ArcGIS Server is that to this day it still responds with HTTP 200 to failed requests ...