本文通过示例对比了SQL中Union与Union All的区别。Union All保留所有记录，包括重复项；而Union则移除重复记录，只显示唯一项。这对于理解如何在SQL查询中处理重复数据非常有用。
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
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