创建一个包含所有数字的临时表，然后左键联接基表并选择所有的空值或顶值，例如

Create Table #Tmp
([SINFO] [nvarchar](50) NULL)

declare @ddd int =74000 
WHILE @ddd < 74015 
BEGIN  
   select @ddd= @ddd +1
   insert into #Tmp (SINFO)  values (  'G' + Convert(Nvarchar ,  @ddd ))
END;  

--all gaps
select  #Tmp.SINFO , Table_2.SINFO  from #Tmp
left join Table_2 on #Tmp.SINFO = Table_2.SINFO 
where Table_2.SINFO is null
order by   #Tmp.SINFO

--Frist gaps
select top(1) #Tmp.SINFO , Table_2.SINFO  from #Tmp
left join Table_2 on #Tmp.SINFO = Table_2.SINFO 
where Table_2.SINFO is null
order by   #Tmp.SINFO

If(OBJECT_ID('tempdb..#Tmp') Is Not Null)
Begin
  Drop Table #Tmp
End