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select null 字段 from
select null+1
selectsum(null)from
select ifnull(null,0)
select null as
select(null<=>null)is null
select isnull(a,b,1)
select length(null)
select (5+null)+3 from dual
select * from where 字段 in
select ifnull(6>2,5>3)
select froma,b
select a.*from
select not null
select 字段 into 字段
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