ORA-00923:FROM keyword not found where expected(単一二重引用符)

1、前言今日oracleを勉強して1つの間違いに出会って、1つの2重引用符についての小さい問題で、以前気にしたことがなくて、ここに記録します.メモ
2、具体的な問題
テーブル構造文は次のとおりです.

create table demo(id int,name varchar(20),nums int);  ----    

クエリ文は次のとおりです.

select sum(decode(name,'  ',nums,0)) as '  ',
       sum(decode(name,'  ',nums,0)) as '  ',
       sum(decode(name,'  ',nums,0)) as '  ',
       sum(decode(name,'  ',nums,0)) as '  ' from demo d

        ，               ：ORA-00923: FROM keyword not found where expected，    ？         

select sum(decode(name,'  ',nums,0)) as "  ",
       sum(decode(name,'  ',nums,0)) as "  ",
       sum(decode(name,'  ',nums,0)) as "  ",
       sum(decode(name,'  ',nums,0)) as "  " from demo d

        as         ，             。             ，         ：

select sum(decode(name,'  ',nums,0)) as 'A',
       sum(decode(name,'  ',nums,0)) as 'B',
       sum(decode(name,'  ',nums,0)) as 'C',
       sum(decode(name,'  ',nums,0)) as 'D' from demo d

        ，                      ，            ，           ：：

select sum(decode(name,'  ',nums,0)) as   ,
       sum(decode(name,'  ',nums,0)) as   ,
       sum(decode(name,'  ',nums,0)) as   ,
       sum(decode(name,'  ',nums,0)) as    from demo d

直接入れないのが一番いい方法で、間違いはありません.上の問題を記録するだけで、後でめくるのに便利です...