最近在看C#方面的書，正好看到迭代器這塊，裡面有個查詢素數的示例我覺得還不錯，於是就想到SQL也來實現一把。

其實有很多種方法，我大概列下下面。結果就只貼一個好了：

方法1： 最簡單粗暴的演算法

DECLARE @i INT ,
    @j INT ,
    @r INT
SET @i = 2
WHILE @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：來自葉子，用平方根計算，比方法1提升了一點效率

--定義一個表變數，用來儲存找到的素數
DECLARE @t TABLE ( id INT )
DECLARE @i INT SET @i = 2
WHILE ( @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：

藉助系統表 還是用平方根

DECLARE @i INT
SET @i = 100  

SELECT  A.number
FROM    master..spt_values AS A
WHERE   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
 

最後附上C#解決辦法：用控制檯程式解決

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();
        }
    }
}