Firebird的iif语句

1、iif  IIF (, ResultT, ResultF) 

示例：

select iif( sex = 'M', 'Sir', 'Madam' ) from Customers

SELECT FIRST 500 "ID","姓名",iif("year" is null,123,"year")
FROM "t1"
 若为null则改为123

2、nullif  NULLIF (, ) 

示例：

select avg( nullif(Weight, -1) ) from FatPeople
这个没测试出来怎么用