To retrieve a list of contents posted by a specific user, set a condition in the query where the user table's 'id' column matches a specific value. For example, if you set the id to 3 and remove the semicolon, the query will return all content posted by the user with id 3. When id is set to 2, no results are found because that user hasn't posted any content. Changing the condition to id equals 4 will list all content from the user with id 4.