查询出多条记录取一条
举一个简单的例子(查询出多个记录任意取一条)：
现有表aaa，
A     B
aaa   1
aaa   2
bbb   3
bbb   4
bbb   5
bbb   6
ccc   7
ccc   8

执行select t.* from aaa t where t.b = (select b from aaa where a = t.a and rownum<2)的结果如下：
A    B
aaa  1
bbb  3
ccc  6

下面有好多方法，来自一位高手列举的：

/*  数据如下：  name val memo  a    2   a2(a的第二个值)  a    1   a1--a的第一个值  a    3   a3:a的第三个值  b    1   b1--b的第一个值  b    3   b3:b的第三个值  b    2   b2b2b2b2  b    4   b4b4  b    5   b5b5b5b5b5  */  --创建表并插入数据：  create table tb(name varchar(10),val int,memo varchar(20))  insert into tb values('a',    2,   'a2(a的第二个值)')  insert into tb values('a',    1,   'a1--a的第一个值')  insert into tb values('a',    3,   'a3:a的第三个值')  insert into tb values('b',    1,   'b1--b的第一个值')  insert into tb values('b',    3,   'b3:b的第三个值')  insert into tb values('b',    2,   'b2b2b2b2')  insert into tb values('b',    4,   'b4b4')  insert into tb values('b',    5,   'b5b5b5b5b5')  go    --一、按name分组取val最大的值所在行的数据。  --方法1：  select a.* from tb a where val = (select max(val) from tb where name = a.name) order by a.name  --方法2：  select a.* from tb a where not exists(select 1 from tb where name = a.name and val > a.val)  --方法3：  select a.* from tb a,(select name,max(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name  --方法4：  select a.* from tb a inner join (select name , max(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name  --方法5  select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name  /*  name       val         memo                   ---------- ----------- --------------------   a          3           a3:a的第三个值  b          5           b5b5b5b5b5  */    --二、按name分组取val最小的值所在行的数据。  --方法1：  select a.* from tb a where val = (select min(val) from tb where name = a.name) order by a.name  --方法2：  select a.* from tb a where not exists(select 1 from tb where name = a.name and val  < a.val)  --方法3：  select a.* from tb a,(select name,min(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name  --方法4：  select a.* from tb a inner join (select name , min(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name  --方法5  select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val  < a.val) order by a.name  /*  name       val         memo                   ---------- ----------- --------------------   a          1           a1--a的第一个值  b          1           b1--b的第一个值  */    --三、按name分组取第一次出现的行所在的数据。  select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name  /*  name       val         memo                   ---------- ----------- --------------------   a          2           a2(a的第二个值)  b          1           b1--b的第一个值  */    --四、按name分组随机取一条数据。  select a.* from tb a where val = (select top 1 val from tb where name = a.name order by newid()) order by a.name  /*  name       val         memo                   ---------- ----------- --------------------   a          1           a1--a的第一个值  b          5           b5b5b5b5b5  */    --五、按name分组取最小的两个(N个)val  select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val  < a.val ) order by a.name,a.val  select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val) order by a.name,a.val  select a.* from tb a where exists (select count(*) from tb where name = a.name and val  < a.val having Count(*)  < 2) order by a.name,a.val  /*  name       val         memo                   ---------- ----------- --------------------   a          1           a1--a的第一个值  a          2           a2(a的第二个值)  b