00022.OracleUnion all 与union分析

Union all:
select a from
(
select 'a' as a from dual
union all
select 'b' as a from dual
union all
select 'a' as a from dual
)

结果

1        a

2        b

3        a

 

Union :

select a from
(
select 'a' as a from dual
union
select 'b' as a from dual
union
select 'a' as a from dual
)

结果

1        a

2        b