case when 自定义排序时的使用
根据 case when 新的 sort字段排序

    case 
    when t2.status = 4
    and t2.expire_time>UNIX_TIMESTAMP()
    and t2.expire_time<UNIX_TIMESTAMP(DATE_ADD(NOW(),INTERVAL 60 day)) then 4
        when `status` = 2  then 3
        when `status` = 3 then 2
        when t2.status = 4
        and t2.expire_time>UNIX_TIMESTAMP(DATE_ADD(NOW(),INTERVAL 60 day))
        and t2.expire_time<UNIX_TIMESTAMP(DATE_ADD(NOW(),INTERVAL 1 year)) then 1
    else 0
    end sort     

     $query->orderBy('sort desc ,t2.expire_time desc,t2.created_at desc');

当colume 与condition 条件相等时结果为result

case colume 
when condition then result
when condition then result
when condition then result
else result
end

当满足某一条件时，执行某一result
```

case
when condition then result
when condition then result
when condition then result
else result
end

• 当满足某一条件时，执行某一result,把该结果赋值到newcolumnname 字段中

case
when condition then result
when condition then result
when condition then result
else result
end new_column_name
```