SecondHandSongs
SecondHandSongs (or Second Hand Songs) is a collaborative website that maintains a global database of mainly cover versions of original works. It also contains information about adaptations and samples. The website allows performers and curators to add songs and update their metadata. It includes links to freely accessible recordings of the covers, and external identifiers for those works and performances in other databases. As of 2021, it included roughly a million covers of 100,000 original works, and was cross-referenced by MusicBrainz. Data and uses. Data are contributed and edited by the active community, so the exact size of  the database has changed over time. In 2007, the project included 60,000 covers. As of 2020, it had reached a million covers. Data schema and identifiers. SecondHandSongs includes a work ID for each work, and a performance ID for each cover of a song by a performer. A work is an equivalence class of performances of the same underlying song. Each performer has at most one performance of a work in the database. Derived datasets. In 2011, the Million Song Dataset project released a SecondHandSongs subset (an intersection of SHS and MSD data). At the time, this was the largest  dataset of cover songs available for academic research. Later, it released the "SHS100k" dataset for machine learning, with 100k covers of 10k works. This has since become a benchmark for cover-song identification.