document questions

To query documents in MongoDB, use the `find()` method. The basic syntax is `db.collection.find({query})`. For example, `db.users.find({age: 30})` retrieves all users aged 30. You can also use conditions like `$gt`, `$lt`, and `$in` for advanced filtering.

To filter documents with multiple conditions in MongoDB, combine conditions using logical operators like `$and` and `$or`. For example, `db.users.find({$and: [{age: {$gt: 20}}, {city: 'New York'}]})` retrieves users over 20 years old living in New York.

A well-formed XML document adheres to the basic syntax rules of XML. These include the following: 1. It must have a single root element. 2. All elements must be properly closed. 3. Tags must be properly nested. 4. Attribute values must be enclosed in quotes. 5. It is case-sensitive. A well-formed XML document can be parsed correctly by an XML parser. 

 <book>
  <title>XML Basics</title>
  <author>John Doe</author>
</book>