Test
<form id="queryForm">
<label><input type="checkbox" name="category" value="Category:Purp"> Purp</label>
<label><input type="checkbox" name="category" value="Category:Arch"> Arch</label>
<label><input type="checkbox" name="category" value="Category:Source"> Source</label>
<input type="button" onclick="buildQuery()" value="Abfrage erstellen">
</form>
<script> function buildQuery() {
var checkboxes = document.querySelectorAll('input[name="category"]:checked');
var query = "Die Abfrage enthält eine leere Bedingung.";
document.getElementById("queryResult").innerHTML = query;
} </script>