Question: 1. what is the difference between isnull and coalesce?
Answer: ISNULL takes only 2 parameters whereas COALESCE takes a variable number of parameters. The main difference is, that COALESCE is ANSI-Standard, so you will also find it in other RDBMSs, the other difference is you can give a whole list of values to be checked to COALESCE whereas to ISNULL you can only pass one.