how to assign a javascript variable to a jsp string variable?

You cannot do this. The JSP statement is executed server-side, before the execution of the javascript statement, that is executed client-side after the browser received the http response. It is not clear your goal, but if you only need to display in the page the value of a javascript variable, you can use:

• trivial javascript:

  document.write(s);

• targeting existing element:

  document.getElementById('myElementId').innerHTML = s;

• using JQeury:

  $('#myElementId').html(s);

If i understand correctly you are trying to build and execute a sql query based on user input that is handled by javascript. As ADC said this can not be done since jsp is executed server side therefore before browser executes javascript. What you can do is create the sql query and pass this as a parameter a different jsp/servlet (or the same if you can handle this case) which will execute the query

In the first page where sql statement is constructed in variable s you should put something like this

<script>
var s = "hello world";
function createLink(){
    document.getElementById('mylink').href=  'page2.jsp?statement='+s;
}
window.onload=createLink; 

</script>

<a id="mylink" href=""/>Click to exexute query</a>

which create a link to you second page (page2.jsp) passing the statement as parameter.

Now in page2.jsp you should retreive the parameter value like

<% String statement = request.getParameter("statement") %>

and then execute your query.

Even better you should use a servlet instead of a jsp page to perform the query. You could read a tutorial for jsp/sevlets to see how this can be done eg. http://www.laliluna.de/articles/posts/first-java-servlets-jsp-tutorial.html