Every financial dataset eventually hits the same wall: the same real-world thing appears under different names, tickers, and codes. Apple is 'Apple Inc.', AAPL on Nasdaq, ISIN US0378331005, CUSIP 037833100, LEI HWUPKR0MPOU8FGXBT394 — and AAPL means something else entirely on another venue's options feed. Merging two datasets without resolving entities silently corrupts both.
The identifier landscape is layered, not redundant. LEI identifies the legal entity (who). ISIN identifies the security (what). The MIC identifies the venue (where). Tickers are venue-local shorthand — convenient and dangerously ambiguous. FIGI, an open standard, identifies an instrument at venue level and is free to use, unlike CUSIP and SEDOL, which are licensed.
The expensive insight of the data industry is that relationships matter more than attributes: issuer → security → listing → venue → jurisdiction. Once those edges exist, everything else (prices, filings, news) can hang off them. This is why 'knowledge graph' is not a buzzword in finance — it is literally the shape of the domain.
A practical rule for builders: store every identifier you encounter, pick one internal id you control, and treat all external ids as claims with provenance, not truth. Identifier mappings drift — companies merge, relist and rename — so resolution is a process, not a table.