SQL | Looping exercise | Find first n primes using looping

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You can learn how to do this exercise if you attend the course listed below!

Software ==> SQL  (198 exercises)
Version ==> Any version of SQL Server
Topic ==> Looping  (3 exercises)
Level ==> Harder than average
Course ==> Advanced SQL

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Your task is to write a query to show the first (say) 1000 primes.  Here's a suggested algorithm:

  • Create an integer variable to hold the value whose primeness you're testing
  • Use WHILE to loop until this integer is 1000, incrementing its value by 1 each time round the loop (let's call this number P)

Within this outer loop:

  • Find out the square root of the number you're testing (use SQRT)
  • Find out what the highest integer is just below this (use FLOOR) - let's call this X
  • Try dividing each number from 1 up to and including X into P
  • If we ever find a number such that P % X = 0 (ie that P leaves a remainder of 0 when divided by X) we have a non-prime, and we should flag this accordingly and break out of the loop

If it helps, here's what you should see to start with:

SQL exercise - Looping (image 1)

The first few primes!

If you get this working, try incorporating a timestamp to see how long it took to run:

SQL exercise - Looping (image 2)

The start of the time test - at the end, set @EndTime and use DateDiff(ms, @StartTime, @EndTime) to work out the difference in milliseconds

Save your hard work as Printing primes.sql, then close it down!

You can unzip this file to see the answers to this exercise, although please remember this is for your personal use only.
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ShukiMolk   Reply to ShukiMolkReply
23 Apr 20 at 02:04

Holy ****!
That was awesome!

It took me waaaaay more time than I care to admit, but I got it.

I had to exclude 2 and 3 because of the use of FLOOR(SQRT(@P)) though. 

There it is:

            -------------------------------------------------
            -- A little script to see how long it takes: 
            DECLARE @StartTime DATETIME
            DECLARE @EndTime DATETIME
            -- This is the start point. At the end we'll set the end point and calculate it
            SET @StartTime = CURRENT_TIMESTAMP
            -------------------------------------------------

/* Printing all prime numbers from 1 to 1,000 */

DECLARE @P INT                            --A variable to hold the number in check
DECLARE @counter INT                    -- A variable to hold the running numbers from 1 to @SQP
DECLARE @SQP INT                        -- the square root of @P (without the leftover)

SET @P = 1
SET @counter = 1

WHILE @P <= 1000                        --As long as @P <= 1000, do the following:
    BEGIN
    /* 1 */ SET @SQP = FLOOR(SQRT(@P))
    /* 2 */ IF @P = 1
                BEGIN
                    PRINT '1 is a special number. it''s neither Prime nor Composite Number'
                END
    /* 3 */ IF @P IN (2,3)
                BEGIN
                    PRINT @P
                END    
            ELSE           -- Now we're REALLY starting to check the number in @P
                BEGIN
                    WHILE @counter <= @SQP
                        BEGIN
                        /* 1 */ IF @P % @counter = 0
                                    BEGIN
                                    /*    PRINT 'Not Prime' */
                                        BREAK -- We can stop checking
                                    END
                                ELSE
                                    BEGIN
                                        IF @counter = @SQP --That means it's a Prime
                                            BEGIN
                                                PRINT @P
                                                BREAK -- We can stop checking
                                            END
                                    END
                        /* 2 */ IF @counter = @SQP
                                    BEGIN
                                        PRINT @P
                                        BREAK -- We can stop checking
                                    END
                                ELSE
                                    BEGIN
                                        SET @counter = @counter+1
                                    END
                        END
                END
    /* 4 */BEGIN
                SET @P = @P+1
                SET @counter = 2
            END 
    END
    ----------------------------------------
    -- Calculating how long it took:
    SET @EndTime = CURRENT_TIMESTAMP
    PRINT ''
    PRINT 'This process took '+ CAST(DateDiff(ms, @StartTime, @EndTime) AS VARCHAR) + ' milliseconds'
    ----------------------------------------

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