Add : element query operator

Author: andrijunaedi

scripts/query-element.js

@@ -0,0 +1,20 @@
+// select * from products where category is null
+db.products.find({
+    category: {
+        $exists: false
+    }
+});
+
+// select * from products where type(category) = "string"
+db.products.find({
+    category: {
+        $type: "string"
+    }
+});
+
+// select * from products where type(price) in ("int", "long")
+db.products.find({
+    price: {
+        $type: ["int", "long"]
+    }
+});