CASE语句是否会丢失PostgreSQL中的别名范围？

Question

首先,这个SQL有效:

select
    case
        when s.luserid > 0 then u.szusername
        when s.lgroupid > 0 then g.szgroup
        when s.lldapid > 0 then 'LDAP Group'
    end as name
from security s
left join users u on s.luserid = u.id
left join usergroups g on s.lgroupid = g.id
order by name

上面的块通过别名name工作证明了排序,并且name当术语name是保留字时声明别名起作用,这与问题无关

当我在其中创建带有别名的case语句时,我的问题就出现了:

注意别名 useid

select
    case
        when sa.luserid > 0 then sa.luserid
        when sa.lgroupid > 0 then sa.lgroupid
        when sa.lldapid > 0 then sa.lldapid
    end as useid,
from security s
left join users u on s.luserid = u.id
left join usergroups g on s.lgroupid = g.id
order by
        case
            when 'user selection' = 'all objects by user' then useid
            else s.lobjectid
        end

在user selection运行SQL之前,文本将由具有文字文本的解析器替换.别名useid和s.lobjectid类型都是bigint.

抛出错误when 'user selection' = 'all objects by user' then useid.

我是否useid在CASE声明中丢失了别名的范围？ 当我尝试在useid这里使用别名时,为什么这会失败.

顺便说一句,这个SQL也可以:

select
    case
        when s.luserid > 0 then u.szusername
        when s.lgroupid > 0 then g.szgroup
        when s.lldapid > 0 then 'LDAP Group'
    end as name
from security s
left join users u on s.luserid = u.id
left join usergroups g on s.lgroupid = g.id
order by
        case
            when s.lobjectid > 0 then s.lobjectid
            else s.luserid
        end

上面的块证明CASE声明中的ORDER BY声明确实有效.关于上述SQL块的逻辑操作的所有争论都与问题无关,因为它只是垃圾示例SQL.

Answer 1

在Postgresql中你不能尝试做什么,因为它不允许你ALIAS在同一个查询中使用一个字段.与Mysql不同,你可以在那里做到.

要解决您的问题,您可以将查询创建为子查询,然后您的别名将是一个字段,因此可以用作:

select useid, lobjectid from (
  select
      case
          when sa.luserid > 0 then sa.luserid
          when sa.lgroupid > 0 then sa.lgroupid
          when sa.lldapid > 0 then sa.lldapid
      end as useid,
      lobjectid
   from security s
  left join users u on s.luserid = u.id
  left join usergroups g on s.lgroupid = g.id
  ) as t
order by
        case
            when 'user selection' = 'all objects by user' then useid
            else lobjectid
        end

或者你可以重复entiry case块

  select
      case
          when sa.luserid > 0 then sa.luserid
          when sa.lgroupid > 0 then sa.lgroupid
          when sa.lldapid > 0 then sa.lldapid
      end as useid,
      lobjectid
   from security s
  left join users u on s.luserid = u.id
  left join usergroups g on s.lgroupid = g.id
order by
        case
            when 'user selection' = 'all objects by user' then 
                  case
                      when sa.luserid > 0 then sa.luserid
                      when sa.lgroupid > 0 then sa.lgroupid
                      when sa.lldapid > 0 then sa.lldapid
                  end
            else lobjectid
        end

某些引擎会让您使用选择范围上的字段的订单号,顺序如下:

select a, b, c from sometable order by 1, 2

这意味着该查询将通过领域进行排序a和b