In the fictional stories of Sherlock Holmes, written by Sir Arthur Conan Doyle, Doctor John H. Watson serves as Holmes' assistant. His primary duty is to record the actions of Holmes for later publication.
In 2365, Geordi La Forge invited Data to take part in a holodeck recreation of a Holmes novel. La Forge took on the role of Dr. Watson, while Data played Sherlock Holmes. (TNG: "Elementary, Dear Data")