I have very basic knowledge about ISINs, Tickers, SEDOLs etc. But I am still unable to understand if there is a unique combination of ISIN_Ticker that exists for each security. For example, if I want to identify a security from a particular exchange and type of security, should I assume that the ISIN_ticker will identify me the security? Or, does the combination of ISIN and ticker make NO sense?
ISIN and ticker are usually different ways of representing the same information with ISIN being supposedly an international standard whereas tickers are provider (such as Bloomberg code or RIC) or market (such as XIST or XEUR) specific. In general an ISIN is a good way of uniquely determining equity instruments but is not sufficient to determine a derivative as, in many cases, the ISIN for the derivative only represents the underlying security and there are multiple derivatives for the same ISIN. Ticker symbols such as BBV-C/X17 (future) or FESA/P/22.23 (option) are used to uniquely identify the derivative. In come cases ISIN + maturity code + strike price are also used but this is uncommon.
Additionally not all US securities have an ISIN but this is becoming less of an issue.
Note that to completely identify an exact security you will need to have ticker/ISIN, MIC/Market code, and currency as some instruments trade on multiple markets with the same ISIN and different (or the same) currency.