Consulta de selección de Mysql basada en múltiples entradas de usuario

Prueba esto:

    var sql = 'SELECT * FROM table';
    var where = ' WHERE';

    //category is present in userinput 
    if(category){
      sql += where + ' category=category';   
      where = ' and';
    }

    //subject is present in userinput
    if(subject){
      sql += where + ' subject=subject';
      where = ' and';
    }

    if(material){
       sql += where + ' material=material';
       where = ' and';
     }

    if(medium){
      sql += where + ' medium=medium';
    }

    db.query(sql,function(error,result){
         console.log(result);
    });