* Mandarin is the official language of China: Almost everyone in Beijing will speak Mandarin to some degree.
* There are different dialects: Even within Mandarin, there are variations in pronunciation and vocabulary.
* Other languages exist: While Mandarin is dominant, some people in Beijing might speak other languages like English or minority languages.
* Data limitations: It's impractical to conduct a census of everyone in Beijing asking about their proficiency in Mandarin.
However, we can say with near certainty that:
* The vast majority of people in Beijing speak Mandarin.
* Mandarin is the primary language of communication in the city.
If you are looking for specific data, you might find information about the prevalence of Mandarin proficiency among different age groups or demographics in Beijing.