SQL查询效率：100w数据查询只需要1秒钟

内存:  G
os: windows
数据库: ms sql server
目的: 查询性能测试,比较两种查询的性能SQL查询效率 step by step-- setp .
-- 建表
create table t_userinfo
(
userid int identity(,) primary key nonclustered,
nick varchar() not null default '',
classid int not null default ,
writetime datetime not null default getdate()
)
go-- 建索引
create clustered index ix_userinfo_classid on t_userinfo(classid)
go-- step .declare @i int
declare @k int
declare @nick varchar()
set @i =
while @i<
begin
set @k = @i %
set @nick = convert(varchar,@i)
insert into t_userinfo(nick,classid,writetime) values(@nick,@k,getdate())
set @i = @i +
end
-- 耗时 : ，需要耐心等待-- step .
select top  userid,nick,classid,writetime from t_userinfo
where userid not in
(
select top  userid from t_userinfo order by userid asc
)-- 耗时  秒 ,够长的-- step .
select a.userid,b.nick,b.classid,b.writetime from
(
select top  a.userid from
(
select top  userid from t_userinfo order by userid asc
) a order by a.userid desc
) a inner join t_userinfo b on a.userid = b.userid
order by a.userid asc-- 耗时  秒，太快了吧，不可以思议-- step  where 查询
select top  userid,nick,classid,writetime from t_userinfo
where classid =  and userid not in
(
select top  userid from t_userinfo
where classid =
order by userid asc
)
-- 耗时  秒-- step  where 查询
select a.userid,b.nick,b.classid,b.writetime from
(
select top  a.userid from
(
select top  userid from t_userinfo
where classid =
order by userid asc
) a order by a.userid desc
) a inner join t_userinfo b on a.userid = b.userid
order by a.userid asc-- 查询分析器显示不到  秒.查询效率分析：
子查询为确保消除重复值，必须为外部查询的每个结果都处理嵌套查询。在这种情况下可以考虑用联接查询来取代。如果要用子查询，那就用EXISTS替代IN、用NOT EXISTS替代NOT IN。因为EXISTS引入的子查询只是测试是否存在符合子查询中指定条件的行，效率较高。无论在哪种情况下,NOT IN都是最低效的。因为它对子查询中的表执行了一个全表遍历。建立合理的索引,避免扫描多余数据，避免表扫描！
几百万条数据，照样几十毫秒完成查询