-----------查询数据库素数的N种方法（包括C#的）--------------

来源：互联网发布：北京青年政治学院知乎编辑：程序博客网时间：2024/06/11 09:06

最近在看C#方面的书，正好看到迭代器这块，里面有个查询素数的示例我觉得还不错，于是就想到SQL也来实现一把。

其实有很多种方法，我大概列下下面。结果就只贴一个好了：

方法1：最简单粗暴的算法

DECLARE @i INT ,    @j INT ,    @r INTSET @i = 2WHILE @i < 100     BEGIN        SET @j = 1        SET @r = 1        WHILE @j < @i             BEGIN                IF @i % @j = 0                    AND @i <> @j                    AND @j <> 1                     BEGIN                        SET @r = 0                        BREAK                    END                SET @j = @j + 1            END        IF @r = 1             PRINT @i        SET @i = @i + 1    END

方法二：来自叶子，用平方根计算，比方法1提升了一点效率

--定义一个表变量，用来存储找到的素数DECLARE @t TABLE ( id INT )DECLARE @i INT SET @i = 2WHILE ( @i <= 100 )     BEGIN        DECLARE @j INT SET @j = SQRT(@i)        WHILE ( @j >= 2 )             BEGIN                IF ( @i % @j = 0 ) BREAK SET @j = @j - 1            END        IF ( @j = 1 ) INSERT  INTO @t SELECT  @i        SET @i = @i + 1    END

方法三：

借助系统表还是用平方根

DECLARE @i INTSET @i = 100  SELECT  A.numberFROM    master..spt_values AS AWHERE   type = 'p'        AND number BETWEEN 2 AND @i        AND NOT EXISTS ( SELECT 1                         FROM   master..spt_values AS B                         WHERE  B.type = 'p'                                AND B.number BETWEEN 2 AND SQRT(A.number)                                AND A.number % B.number = 0 )ORDER BY A.number

最后附上C#解决办法：用控制台程序解决

using System;using System.Collections;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace Ch11Ex03{    class Program    {        public class Primes        {            private long min;            private long max;            public Primes(): this(2, 100)            {            }            public Primes(long minimum, long maximum)            {                if (min < 2)                    min = 2;                else                    min = minimum;                max = maximum;            }            public IEnumerator GetEnumerator()            {                for (long possiblePrime = min; possiblePrime <= max; possiblePrime++)                {                    bool isPrime = true;                    for (long possibleFactor = 2; possibleFactor <= (long)Math.Floor(Math.Sqrt(possiblePrime)); possibleFactor++)                    {                        long remainderAfterDivision = possiblePrime % possibleFactor;                        if (remainderAfterDivision == 0)                        {                            isPrime = false;                            break;                        }                    }                    if (isPrime)                    {                        yield return possiblePrime;                    }                }            }        }        static void Main(string[] args)        {            Primes primesFrom2To100 = new Primes(2, 100);            foreach (long i in primesFrom2To100)                Console.Write("{0}\n", i);            Console.ReadKey();        }    }}

0 0