Constant DANGLING_BINDINGS 

pub const DANGLING_BINDINGS: &str = "\
SELECT b.collection, b.link_id, b.source FROM bindings b \
WHERE NOT EXISTS (SELECT 1 FROM items i \
  WHERE i.collection = b.collection AND i.link_id = b.link_id) \
ORDER BY b.collection, b.link_id, b.source";

The bindings whose item is gone: the one dangling row a repair can clear, since nothing can read it (DELETE_DANGLING_BINDINGS).