SELECT
  ID,
  user,
  area_list
FROM
  t_white_user
WHERE
  user = #{ user,
  jdbcType = VARCHAR }
AND (
  COALESCE (area_list, '') LIKE CONCAT (
    '%' ,#{ country,
    jdbcType = VARCHAR }, '%'
  )
  OR area_list IS NULL
)
LIMIT 1

//null只能和is或is not搭配，不能使用=、!=或者<>
select * from student where name is null;
select * from student where name is not null;
//''的判断可以使用=、!=或者<>
select * from student where name = '';
select * from student where name != '';
select * from student where name <> '';

select COALESCE(null,null); //报错
select COALESCE(null,null,now()::varchar,''); //结果会得到当前的时间
select COALESCE(null,null,'',now()::varchar); //结果会得到''
//可以和其他函数配合来实现一些复杂点的功能：查询学生姓名，如果学生名字为null或''则显示“姓名为空”
select case when coalesce(name,'') = '' then '姓名为空' else name end from student;