COALESCE takes any number of arguments and returns the first value that is not null.
COALESCE(x,y,z) = x if x is not NULL COALESCE(x,y,z) = y if x is NULL and y is not NULL COALESCE(a,y,z) = z if x and y are NULL but z is not NULL COALESCE(x,y,z) = NULL if x and y and z are all NULL
COALESCE 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.
SELECT name, party ,COALESCE(party,'None') AS aff FROM msp WHERE name LIKE 'C%'