Compatibility
IFNULL(f1, f2)
Engine	OK	Alternative
ingres	Yes	COALESCE(f1, f2)
mysql	Yes	COALESCE(f1, f2)
oracle	No	COALESCE(f1, f2)
postgres	No	COALESCE(f1, f2)
sqlserver	No	COALESCE(f1, f2)

IFNULL

IFNULL takes two arguments and returns the first value that is not null.

  IFNULL(x,y) = x if x is not NULL
  IFNULL(x,y) = y if x is NULL

IFNULL 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
      ,IFNULL(party,'None') AS aff
  FROM msp WHERE name LIKE 'C%'

SELECT name, party
      ,IFNULL(party,'None') AS aff
  FROM msp WHERE name LIKE 'C%'

SELECT name, party
      ,COALESCE(party,'None') AS aff
  FROM gisq.msp WHERE name LIKE 'C%'

SELECT name, party
      ,COALESCE(party,'None') AS aff
  FROM msp WHERE name LIKE 'C%'