查询时别名

在 mysql 中

1. group by ,having ,order by 支持字段的别名。
   group by ,order by 后使用别名

   select
       case
           when t.author_level in(1,2) then '1-2级'
           when t.author_level in(3,4) then '3-4级'
           when t.author_level in(5,6) then '5-6级'
           else ''
       end as level_cnt,
       count(s.issue_id) as num
   from author_tb t join answer_tb s on t.author_id = s.author_id
   where s.char_len >= 100
   group by level_cnt
   order by num desc

2. where 后面不支持字段的别名。

3. 如果关联查询的时候给表起了别名。那么 where 后面如果用’表名.字段名’的话这个表名要用表的别名

4. case 关键字后面可以接表的别名

   select
   	case  record.xingbie
       when 1 then '上行'
       when 2 then '下行'
       when 3 then '出段'
       when 4 then '入段'
       end as xingbieName
   from
       t_sjfx_mh_jcjl record
       left join sys_dict_single lineDict on lineDict.value = record.xianbie