求SQL：当条件数据为空时，则查询所有字段的集

[发哥]

select * from a

当A中的字段 id is not null时
则查询id条件为 id in(1,2,3,4,5)

应该如何写这条SQL语句？望各位达人给予指导，谢谢。

[lcqtgb]

select * from a
where id is not null and id in (1,2,3,4,5)

[lcqtgb]

if exists (select * from a where id is not null)
select * from a
where id in (1,2,3,4,5)

[王向飞]

select * from a
where id in (1,2,3,4,5)
union all
select * from a
where id is null

[duanzhi1984]

if exists (select * from a where id is not null)
select * from a where id in (1,2,3,4,5)