使用select语句替换sql中的null值？

在MySQL中，有很多选项可以代替NULL值：

CASE

select case     when fieldname is null then '123'     else fieldname end as fieldname from tablename

COALESCE

select coalesce(fieldname, '123') as fieldname from tablename

IFNULL

select ifnull(fieldname, '123') as fieldname from tablename