multi-part index and when it is used?

[Peter Halliday]

Say I have a phonebook table that has a multi-part index on lastname, firstname, middleInitial. If I do SELECT * FROM phonebook WHERE lastName = 'Smith', then it should use the index right? What if I do SELECT * FROM phonebook WHERE lastName LIKE 'S%' would that use the index also? If I wanted to list the whole phonebook would SELECT * FROM phonebook use the correct index if I had multiple indexes?

THe idea behind this is I heard that trying to remove ORDER BY by using indexes helps on RAM and can speed a query up potentially. Therefore, I was thinking of how to do this in scenarios like the the one give where there are some WHEREs, but not all.

Peter