Question

我想先选择10行，然后选择其中的5行。

Answer 1

必须在此代码段中定义“前10个”并且它是“某事”。这适用于SQL Server 2000 +

select top 5 
    * 
from 
    (
    select top 10 * from <table>
    order by something --got to have soemthing here to define "first 10"
    ) as a
order by 
    newid()

编辑：

为什么在派生表中需要ORDER BY

-- large row, same result with and without index/PK
CREATE TABLE #foo (bar int /*PRIMARY KEY NONCLUSTERED (bar)*/, eejit char(8000))

--create rows with value 1-10 + some others
INSERT #foo (bar) VALUES (1)
INSERT #foo (bar)  VALUES (10)
INSERT #foo (bar)  VALUES (20)
INSERT #foo (bar)  VALUES (2)
INSERT #foo (bar)  VALUES (5)
INSERT #foo (bar)  VALUES (45)
INSERT #foo (bar)  VALUES (99)
INSERT #foo (bar)  VALUES (3)
INSERT #foo (bar)  VALUES (9)
INSERT #foo (bar)  VALUES (7)
INSERT #foo (bar)  VALUES (6)
INSERT #foo (bar)  VALUES (4)
INSERT #foo (bar)  VALUES (8)

--create logical fragmentation
DELETE #foo WHERE bar IN (1, 3, 5, 7, 9)
INSERT #foo (bar) VALUES (1)
INSERT #foo (bar) VALUES (3)
INSERT #foo (bar) VALUES (5)
INSERT #foo (bar) VALUES (7)
INSERT #foo (bar) VALUES (9)

-- run this a few times, you will see values > 10
-- "first 10" surely means values between 1 and 10?
select top 5 
    * 
from 
    (
    select top 10 * from #foo
    ) as a
order by 
    newid()

-- always <= 10 because of ORDER BY
select top 5 
    * 
from 
    (
    select top 10 * from #foo
    order by bar --got to have soemthing here to define "first 10"
    ) as a
order by 
    newid()

Answer 2

select * 
from 
    (select * from table order by id limit 10) as rows  order by rand() limit 5

Answer 3

不确定你想要做什么，但这可能会有所帮助（在MySQL中）：

 SELECT col1, col2, ... FROM tablename ORDER BY RAND() LIMIT 5

使用RAND()和LIMIT来实现您的目标。

选择前10行，然后选择它们的5个随机行

3 个答案: