NVL takes two arguments and returns the first value that
is not null.
NVL(x,y) = x if x is not NULL
NVL(x,y) = y if x is NULL
NVL can be useful when you want to replace a NULL value with
some other value. In this example you show the name of the party
for each MSP that has a party. For the MSP with no party (such as
Canavan, Dennis) you show the string None.
